1) I took my UCAT early August (around the 8th August or something like that), and in total I revised around 2 months starting in June. In June, revision was a lot more relaxed as I still had schoolwork, so I only used the official UCAT questions and passmedicine questions, with only a few questions a day. This was mainly to see how I felt about the questions and start getting used to them. Then a month before my test, I got medify and really started to drill questions, at least 2-3 hours a day on each section, maybe 1 full practice paper and then some topic question focus. On the week/few days before the test, I was up to around 2 full practice papers a day at least, so maybe 4-5 hours. It helped that I actually enjoyed the UCAT questions, and enjoyed pushing myself to see how far I could score, so I was able to work on it for long periods of time without getting too overwhelmed.
2) I used medify and got a 1 month access, and also used the official UCAT questions and free passmedicine questions. I know some people use medentry, but I can't say how good it is since I've never tried it, though it seems good as well. I didn't use tutoring sessions, and I think there are a few somewhere if you search, but in my opinion they don't seem worth the money, especially if you are decent at self learning and can look at past questions and identify mistakes. They were quite expensive if I remember right, same as any private tutoring thing.
3.1) Abstract reasoning was initially the hardest section for me, as the rules are often very obscure and the time limit is insanely low, about 14s per question. I used some sort of Acronym I memorised beforehand to help me identify the rules of each set of questions. There are many acronyms, the most famous is probably SCANS, and I made my own which ended up as something weird like SCOPNSS I think, remembering it as SCOP N S S (for some reason it stuck in my mind). Each letter stood for something, I believe it was Shape, Colour, Orientation, Position, Number, Size, Symmetry, and each word has certain things the rule could be, like different shapes for categories A & B, different colours, direction of arrows, location of a specific shape (e.g. a square always in top left), number of shapes and/or sides and odd/even, biggest shape being a specific thing, and lines of symmetry. Medify did help with this as it gave examples of each thing in the theory section. After this it was just practice and applying this knowledge, and learning exam technique like flagging and guessing, spending about 30 second on the first question of an A/B set as 5 questions in a row will be on the same A/B pattern.
3.2) Verbal reasoning ended up as my worst section, and also the section I liked the least when revising. Time pressure is also bad here, around 30 seconds per question. For me, what worked best was reading the question and then scanning the passage to find which area seems most relevant to the question, then looking at options and figuring out the right one by either finding it outright or ruling others out. Certain questions have specific wording, which I learned from Medify theory section, and that helped identify what to look out for, as well as any modifying words like ONLY or AND (both statements need to be true for the option to be correct). Usually wrong options will not be outright wrong but have a small change, so I was careful for those, as well as the ones which needed you to work out implied stuff. Finally I learned to just move on and guess after 30 seconds, even if I had found 2 wrong options, as it leaves a 50/50 instead of a 25% chance if you don't have time at the end and have to guess the remaining questions. Start with just slowly working out questions with no time limit, then eventually start to put time pressure, as it is pointless putting a time limit if you have no idea how to work out the question. Same applies for all UCAT sections.
4) In June, I focused more on A levels due to end of year exams and also taking Further Maths A level, and only did a little bit of UCAT and focused so much more in July and the summer. Since I did my UCAT in August, that left the rest of August to keep up to date with my A levels, which helped me. However, I did have to then immediately start revising for BMAT (which is sadly discontinued) so there was still some pressure. I was fortunate enough to not need to study that much for A levels (until winter/spring, where I really had to ramp up my studying), so during August I would do a lot more BMAT and then check over A level knowledge. In September exams I took a small break from BMAT, and then did BMAT whenever I had free time after completing homework in the evening/night. Again, it helped that I enjoyed the style of questions so it didn't feel like much of a burden, and fortunately got good results for BMAT without impacting my A level study that much.
It is very possible to get a great UCAT score with bad GCSE grades. UCAT is a completely different style of questions compared to GCSEs, and even quantitative reasoning uses maths knowledge below GCSE standard, mainly arithmetic, graphs/charts and percentages/fractions. Even though I did get great GCSEs, they didn't impact how I did UCAT at all. UCAT is much more focused on problem solving and time management, whereas GCSEs are more knowledge focused and some application, so bad GCSEs will never guarantee a bad UCAT score, just as good GCSEs don't mean you get a good UCAT score. Once you get your UCAT score immediately after the exam, have a look at specific medical schools which take into account UCAT, GCSEs or both. Most will say what GCSEs you need, and if you satisfy those requirements then apply, if not then don't. GCSEs and UCAT are usually only used as a filter for interview, with exceptions like Oxbridge and Edinburgh, so if you meet the bare requirements then you should get an interview, which is where they will decide. For example, despite my UCAT score I was rejected from Liverpool due to a not high enough interview score, as UCAT is only used to shortlist for interview. Just be tactical about where you will apply for medicine, and you will have as good a chance as anyone.
Don't be too pessimistic about GCSEs, they are not the be all and end all. From your post it seems like you are in Year 11 or below, since you haven't gotten your GCSE grades from what you have written (so sorry if I am wrong), so I wish you all the best for it. Volunteering and Work experience can come later, I did most of my volunteering and work experience mainly over Year 12/13 summer so there is still time. If you need work experience it can't hurt to try the virtual work experience held by Medic Mentor, where you can do one every month for free, just be sure to write down what you learn from it, reflect on it, and how it helps you be more prepared for a career in medicine.