Depends where you are flying from.
HMRC leaflet "Carrying cash in and out of hte United Kingdom" refers.
If you are coming from a country outside the EEC the limit is 10,000 euros. Anything over this must be declared.
If you are coming from an EEC country there is no limit BUT " you must answer any questions that an HMRC Officer may put to you about the cash you are carrying".
Jean