northern boy
I'm not really well up on these things, but I'll have a stab at what you could expect.
Join on whatever fleet BA offer. If you have a jet rating, your first type could well be 777. A320 if you prefer shorthaul. After an initial freeze of 4-5 years you have a free bid to the type of your choice, or stay where you are without a freeze.
Your earliest command opportunity is highly likely to be A320 at LGW.
It's anyone's guess how long this could take, but I imagine 7 years or so might be possible. Expect a little longer for an A320 command at LHR.
SH out of LHR is not particularly pleasant. Particularly if you are junior on the fleet, and have a family, or enjoy your weekends.
So if you join at 47-48, you could well see a command by 55, at which time you could work for another 10 years if it suited you.
HTH