Considering that Brits tend to view Canadians as a "lower form of life," I'd say Bombardier is out of the running.
The Avro RJX is the obvious choice for buying British, but it fails to suit their needs. It's designed for STOL, and guzzles more gas with its 4 engines than the others do with their two.
Embraer is the cheapest, but probably not the most technically advanced. There would be full commonality, though.
Fairchild has the 328JET, 528JET, 728JET, and 928JET all out or coming out. These give excellent commonality, and will be extremely advanced, but won't be available for some time yet. If BA's in no particular hurry, this is the best choice.
Don't rule out Boeing, however, as a serious look at the B717-100 by BA just might convince them to talk launch.
And the winner? Probably Fairchild or Embraer. Depends how quickly they want the product.