Hello,
You can be offered either fleet depending on their requirements at the time. It may be possible to delay your joining in order to go to your prefered fleet.
On the 737 the initial base has been Doncaster of late. Plenty of growth means you may get rebased quite quickly. LGW or MAN is quite likely on the 757.
Lowest pay to date £2800ish highest £3800 ish (year 1 FO)
Time to command, jury is still out on this one. I would say a couple of years if you have the hours, a suitable training record and the expansion continues; otherwise it may be a fair bit longer.
In anycase a great outfit where they really look after you.
I hope it helps.