I haven't used TCs in over 20 years in the USA or in most of the other places I have visited. (Thailand being an exception)

As said above even if you don't have Nationwide and pay the full 2.75% on your 'plastic' you still get a better exchange rate than TCs and you can get your Airmiles/cashback with credit cards.

If you have enough money to buy TCs in UK, then why not use that money with a debit card for cash from an ATM or purchases and you won't "have too much of a bill when you get home".