Thats going to be fleet dependent because it depends how many people get rercruited below you. LGW 319 would be good if you joined in the first batch (despite carmen) because although you will still be bottom of the airline list, within that fleet you'll be top as its a new fleet and there are no internal moves to it. A320 LHR has a more constant turnover so you'd creep up relatively quickly. 737 LGW would be worst option at the moment as you could end up stuck as the most junior pilot on a shrinking fleet.