I just completed an Easter placement at Slaughter & May, one of London's five "magic circle" law firms, which was awesome. What stage are you currently at, 6th form / uni ? Because all of these placements open up in your penultimate (usually 2nd) year - all top firms offer standard placements, which are incredibly competitive and pay about £250/week. Otherwise, buy the Chambers Student Guide for a tenner which gives you all the law firms which offer work experience; you have a better chance in smaller firms. And if you're interested in going to 'the bar' (the legal, not alcoholic one) then try mini-pupillages - a lot easier to get accepted onto as they don't pay.