I think this order is only going to be for widebodies and it will be a case of A380s/A350s -v- 748s/773ERs/787s. As you point out, BA is going to drive an extremely hard bargain.
Whether or not there will be any replacements for the RJ100s (which were not included in the Flybe contract), depends on how routes out of LCY do. Even then, the A318 might not be the ideal choice.
Personally, I see BA retreating to be a Heathrow based airline over the next few years, even leaving LGW - with limited service being operated by GB Airways.