It's very simple, really. It boils down to money, as always. I suspect these aircraft were cheap to acquire and as they fit up to 174 seats BA would want to utilise this as much as possible. Every short haul aircraft at LGW is capable of doing every route. The company doesn't care about whether the crew have somewhere to "hide" or not. The only thing that speaks is customers not booking flights.

Also, we don't have any "better" aircraft at LGW, unless we have a borrowed visitor from LHR. It's either A32E (as we call them - no bulkhead) or the ex-BMI A319s which are also unsuitable for the longer flights due to only 2 toilets.