Police can seize any amount over 1 thousand pounds if you can not give a good reason carrying it.
HMRC officers will not detain properly declared cash if they have no reason to doubt its legitimacy. However, as with cash in amounts of £1000 or more found at any place within the UK, under the Proceeds of Crime Act 2002 (POCA), it may be seized if an officer has reasonable grounds to suspect it is either the proceeds of, or is intended for use in, unlawful conduct.