Are there legal requirements for transiting in Heathrow with €10,000 cash?

Posted by upupupdo@reddit | AskUK | View on Reddit | 9 comments

Old schools in-laws will be passing €10,000 cash as gift to daughter who will be flying from Madrid with BA, via Heathrow, to North America. Are there any legal requirements on holding that amount of cash while transiting in Heathrow?