Pret makes a lot of sense, kind of generic/broad appeal but good enough quality that people will pay for it and doesn't clash with the attempt to hang on to some semblance of 'premium' in ET. Also has the advantage of being a known brand to connecting passengers from the US/Middle East, etc.

Greggs would be completely incomprehensible on so many levels (not premium, unknown outside of UK, it even has the wrong geographic focus for most of BA's customer base in the UK). 'Hi Sir/Madam, you look hungry after your long connecting flight from Jeddah/Muscat/Kuala Lumpur/Lahore/Tel Aviv, could I interest you in a sausage roll?'