As far as I know Travelex indeed have a monopoly at T5. LGW - to help anyone else - has a nifty way of having 2 providers, ICE is actually on the curtilage of the railway station so not under GAL's control, who currently have the Moneycorp franchise.

That also applies for T2 / T3 in that Heathrow tube station has ICE. Plus myriad options at Paddington if using HEX.

I used to be religious about thinking about currency on arrival, With now only a few exceptions I'm relaxed to go around with some USD $5 notes in pristine condition, the Post Office credit card, Nationwide and Revolut. Between them I've never been caught out (though Germany is probably the riskiest even now!).