Resorting for clarity with personal observations added:
Marco clearly stated the double conversion whammy. You only get currency within the country which issues it.
Some airport concessions will accept other currencies, but at an unfavorable rate.
Heathrow machines are TravelEx.
WHSmith will accept a credit card for a magazine as will Nero for a shot of coffee. It would be reasonable to assume that other concession would do the same. Worst case is a two percent FTF.
I can picture bank ATMs at BRU.
Not long ago I entered through Heathrow with, stupidly and unknown, only Scottish notes in my stash. I grabbed fifty pounds from a TE machine and two hundred more from a bank machine a couple of hours later. I paid an extra seven or eight bucks for the first bit of money. TravelEx machines in other nations sometimes have an extra usage fees on top of the sorry exchange fee.
Heathrow isn't my usual departure airport and the last one was a mad dash. However, both Turnhouse and Manchester only have TravelEx/Amex machines and booths on the arrivals level but do have bank machines prior to security on the departure level. This May or may not be the case at Heathrow.