The premier in T5, Sheraton and HGI were all open during the national lockdown(s) and will remain so during tier 3. Why BAH are saying that is beyond me, that said they are a separate entity to BA. I’ve been actively using all three throughout this year. I have a feeling a few members of staff in a certain area have either been given duff info, or are speaking out of turn.
With ever shifting goal posts it is hardly surprising the odd duff gen gets passed on unwittingly. If you have a need to travel you are perfectly entitled to use your booking as per the rules ‘guv. That said I have never seen the Stasi officers of our newly created East Germany doing any anti-mask hunting operations in any hotel this year, so no one will ask and the hotel will be genuinely glad to see you.
Happy travels.