The top law schools in the UK are about as competitive as the top schools in the US. Cambridge and Oxford want a 3.75 GPA minimum (from University and usually high school, as well) plus high LNAT scores. (Not all schools even require the LNAT, as it is only a recent test to be administered to prospective law students. LSAT scores are not accepted. The LNAT is also much easier.) King's looks for a minimum 3.7 and high LNAT. UCL looks for a minimum 3.5 and high LNAT. Durham looks for a minimum 3.3 and high LNAT. LSE looks for a minimum 3.2. But, remember, these are minimums. It's very rare to get an offer by just meeting the minimum requirements. In the UK, Universities have minimum requirements and students do not apply unless they meet the minimum requirements, as you can only apply to 5 Universities. There are still around 7 to 10 applicants per place at the top law schools and every one them will be a top student.
Acceptance rates for law schools in the UK are also about equal to those in the US. However, UK law schools like international students because they pay international rates (around 12,000-13,000GBP per year). Remember though that law in the UK is an undergraduate program and earns one an LLB, not a JD.