I've been thinking about doing the same actually. Law's quite a popular degree so there will be competition for places at Universities like LSE, UCL and King's, so a gap year would be your best bet for that. There are always opportunities in Clearing and Adjustment for Law LLB courses, but a place at a top, top University like those mentioned would be difficult in Adjustment and Clearing. What I would recommend is to take a gap year and do Work experience within that year, like my Law teacher was telling me about a scheme where you can Shadow a Judge at your Local Crown Court - This would be pretty impressive on your Personal Statement. Alongside this, you could also apply to become a Lay Magistrate for the year, you'd have to do at least 28 half days in the year and it's not guaranteed you'd be picked, but you don't need any Legal Qualifications for it and Universities would love it.
As for your chances of becoming a Barrister or Solicitor in the future, there's a pretty good chance. To become a Barrister, after you get your degree you'll need to pass the Bar Training Course and then go through a Pupilage (where you shadow a qualified Barrister for 12 months) and then you'll be called to the Bar and become a Barrister, so it wouldn't take too long. Solicitor wise, it's a similar process: after getting your degree you'll need to pass the Legal Practice Course before getting two years' worth of practical experience in either a Solicitor's firm, Crown Prosecution Service or other Legal Organisation. You'll then be admitted as a Solicitor by The Law Society.
Overall, it's a pretty cool profession to go into, it's interesting, pays well, commands respect and you can eventually go on to become a Judge.