There is no legal limit to how much cash you can physically carry into or out of the UK, but there is a strict declaration requirement for large amounts. If you are carrying £10,000 or more (or the equivalent in any other currency) into or out of Great Britain from a country outside the UK, you MUST declare it to UK Customs. If you are traveling to or from Northern Ireland, the rules differ slightly, requiring a declaration for amounts over €10,000. "Cash" includes not just banknotes and coins, but also traveler’s checks and bearer bonds. Failure to declare these funds can result in the entire amount being seized by Border Force and a penalty fine of up to £5,000. Furthermore, even if you declare the money, authorities can seize it for 48 hours if they have reasonable grounds to suspect it is linked to criminal activity. For domestic travel within the UK (e.g., flying from London to Edinburgh), there are no declaration requirements, though carrying massive amounts of cash may still lead to questioning by security.