1) The chances of you getting a place through clearing are quite low, as you probably know - due to the vast number of people with 4 rejections, like yourself, who will want those places. Obviously, give it a go - especially if you get your 3 A*s - but don't get your hopes up. I don't know if those places consider UKCAT/BMAT scores when allocating clearing places - if they do, that may count against you.
2) How to get 3 A*s? I'm sure you know the answer already - revise a LOT, have a good revision timetable to make sure you get through everything, do past papers, sleep well. I'm not an expert but I expect that if someone is offered a place on results day from a uni that rejected them, it's very likely they would have had an interview, so I doubt that route is open to you.
3) Taking a gap year is your best bet. Travelling around definitely isn't what you should be doing though, so don't worry about not being able to afford that. Try and get a job at a local hospital as a healthcare assistant or something, or failing that, volunteer at an old people's home and try and get more medical work experience. To earn money to pay your way, work as a bio/chem/maths GCSE/A-level tutor - it pays well, especially if you have A* at A-level in those subjects. Use tutor hunt to find work. Also, work on improving your UKCAT as much as you can (and BMAT, if you decide to apply to UCL again). Perhaps do one of those UKCAT courses.
Alternatively, you could take your Biomed/Biochem/whatever your fifth option is, and then do graduate entry medicine. Bear in mind though, you will be studying for a lot longer, possibly 7/8 years, so this may be tough financially (although some of those years will involve paid work).
4) I know lots of people who've been in your position and they all got at least one medicine offer the second time round. Some of them were slightly less ambitious in their choices when they reapplied - so went for Hull York, Leicester etc. With medicine, it doesn't matter where you go - the degree is the same and you're guaranteed employment.
My final advice would be, DON'T GO TO PRAGUE OR BULGARIA!!!! My mum is a GP involved in training recent medicine graduates who want to become GPs, and she says that a Prague/Bulgarian medicine degree is nowhere near as good as a UK medicine degree. The Prague graduates she trains really struggle, and one failed the key GP assessment 3 times, whereas all her UK-trained graduates pass first time.