Currency exchange regulations, which strictly limited the amount of cash which travellers could take out of the UK, were abolished many years ago. (There were never any restrictions on bringing currency into the country).
So you can take out, or bring in as much cash as you like. However, if you're travelling to or from a country outside of the EU you're obliged to declare any cash which you're carrying (including cheques, bankers' drafts or travellers' cheques) to HMRC if the amount you have with you exceeds €10,000 (or its equivalent). Such a declaration doesn't prevent you from moving the cash into or out of the country. HMRC can only prevent you from doing so (by seizing it, pending further enquiries) if they have reason to suspect that the money is in some way related to criminal activity:
http://www.hmrc.gov.u...ng/declaring-cash.htm
The same rules apply across the whole of the EU. (i.e. you can move as much money as you like, without any declarations, within the EU but you must declare amounts above €10,000 when entering or leaving the EU).
Chris