Sorry not correct. Amounts over £10,000 must be declared to the cash team of HM customs with all undeclared cash being subject to confiscation. You may be able to appeal the confiscation.
But HMRC also have the right to require you to prove the origins of amounts over £1,000 and if unsatisfied with your answers can require you to prove it’s origins to a court within 30 days or forfeit it, however not declaring it doesn’t automatically get the cash detained specially if you have ATM, bank withdraw slips, ay slips etc or other proof of it not being the proceeds of illegal activities.
You may decide to make a voluntary declaration under £10,000 as the cash team can be friendly and you have more time o it’s less stress. My experience was that during the total time I was with them, about 15 minutes, there was only one other person who made a declaration. YMMV
If it is not a direct flight remember that other countries may have much lower requirements for declaring cash, as I found out on my last flight from the U.K. my transit country had a $3,000 limit, other countries may require that even in transit airside you still have to declare cash.