Agree with that, and point out you can possibly get a slightly more clear picture by going to ba.com rather than a third party site, but no guarantees...
FWIW ATM looking at ba.com Wednesday 28th late AM it appears to me:
On the 8th it looks like BA are still selling on two direct EDI-LHR services, the 0650 and the 1945 departures, but doesn't look like there's a lot of availability.
On the 9th they are selling on four EDI-LCY (London City) services, late AM through mid PM and a single EDI-LHR at 1735...
Health warning - just because they are selling doesn't guarantee the fight will eventually operate...
I wouldn't expect the LCY flights to be effected by LHR capacity issues so they might be worth bearing in mind as backups / alternatives but the timings and any transfer over to LHR may not work for the OP.
TBH if I was the OP I'd be considering cutting out the middle person and talking to BA directly on the phone (if the OP can get through) and at least find out from them what the policy would be with regard to the Longhaul sector if the prior domestic gets binned.....will the OP be able to use it if they get to London in time, or does the whole booking get scrubbed?
If BA say the London-US sector would indeed still be valid if the domestic is cancelled I'd agree in theory at this stage it's best to stick with BA - if they do cancel that sector and let them do any rejigging/pick up any bills.