I've used the 111 PCR letter for international travel from LHR to Amber, 2nd July. No problem with it getting accepted, even though they issue them on Word docs.
You're more likely to run into problems on your return. Not a problem to do the UK Border Passenger Locator form, which you must do, but you might have a problem, as I did, with proving your IOM test to release on return is acceptable in lieu of a UK test to release.
Best bet is to enter your travel details into this:
And read the small print regarding/ignoring the Isle of Man. All international airlines follow the advice of this system (Timatic) to the letter & do not recognise the Isle of Man as a separate entity.
Bookmark the site, you might need to quote its advice at check in.