US/UK before the quarantine
 

I gloomily fear that business will require me to have 3-4 days in London, which I will now need to achieve before the quarantine comes into force. Is there a OneWorld way to get ORD/LHR without enduring the complete non-service currently offered in premium cabins on BA? I'm finding this hard to work out. I'd happily transit through MAD, DOH, DUB or wherever to get more than a sandwich and a bottle of water on the flight.

And if any UK/London based readers could advise - will I find any hotels open in the capital?

Not sure re flights but re hotels many are open but only to essential travellers, you should check directly with the hotels regarding whether your circumstances would be deemed essential according to their policies.

What have found when looking? You will get more answers if list the options you have found and why these are OK-not OK. FT is not a travel agent
Have you considered the transit restrictions, if any, for the flights via a 3rd airport?
Routes ---> Great Circle Mapper

For a short business trip you would consider going via DOH or MAD rather than a short seven hour direct flight and take your own extra snack if needed? Crazy

Fly via DFW or MIA on AA.
American still offers a full service on intercontinental routes.

Another alternative is to fly via DOH but that would consume your time. It's all up to you.

Many/most countries will not let you in/transit. Just fly direct for your sake and others'. I don't think any reasonable routing will offer you champagne and caviar at the moment, all fairly pared back service, in part to minimise contact.

AA does offer their full service at the moment. The OP could easily fly ORD-MIA/DFW-LHR with having the full service on the transatlantic leg without involving any other country.
Also, Qatar does allow transfer passengers.

However, I'm not sure of the policy of Spain ( Iberia ) and Ireland ( Aer Lingus ).