A318 probably won't do at BA because:
1. With the BACon sale the number of Avros to replace is very small and a larger requirement down the road is unlikely.
2. Even if they did want to go Airbus, an IAE engine is not offered on the 318. CFM56 is, but with the BCal A320s and the remaining 737s likely to leave the fleet in due course it's unlikely BA will want to get new frames with CFMs and certainly not PWs.