I'm currently final year A level student and I just took up further maths this year after self teaching myself the content during the summer break.
I don't think it's too late to switch at all and at this stage if you are taking single maths you probably won't be too far behind what the further maths students are doing at your school.
For self teaching my biggest advice to you is to have a good look at the content and actually see if you like any of it. A lot of people will have probably told you this already but its going to be hugely difficult to study something you don't actually like especially if you are self teaching it during your own time. Another thing to consider is that if you do decide to do further maths, half of all your lessons from the moment you switch will be maths.
Once you have decided that you like the content (or not in which case probably don't bother with further maths
) go and ask your teacher what the further maths students have covered and plan accordingly. Set (realistic) targets and deadlines and fully commit to getting it done. A lot of your friends will probably be partying and messing around during this holiday but there probably will be times where you just have to stay home and do some work.
If you are still slightly unsure about your decision but want to go ahead anyway (I was very unsure as well since it is quite a large commitment) try studying the single maths materials that further maths classes have studied first before doing further maths topics and see how you get on.
I found that the best way to self teach further maths content was to look at the formulas they give you first, try some worked examples or look through them, then do practice questions. When I was self teaching further maths about 10 - 15% of the time was spent reading and the rest was spent doing questions. And be sure to leave a little time at the end to go through everything.
As for what further maths is like this is going to depend on what exam board your school uses and what modules you will do. From my experience further maths can be a really fun and challenging subject. I definitely do look foreword to studying a lot of the topics on my syllabus but the pace is usually pretty quick.
As for recommending you the subject. No one can really say. A lot of my friends make sour faces when I tell them I study Maths or Chemistry. Everyone is different, so I'm afraid I can't really recommend anything.
I hope I can help and if you have anymore questions don't hesitate to ask