WTFH
I'm afraid what BYMONEK says is still true to this day. Most months I have roster changes in double figures, even in Winter when things are much quieter. It really does play havoc with arrangements particularly if you have kids and a wife / husband that also works.
As a general guide to your question though as an A320 F/O at LGW expect 15 to 20 flying days per month from May through to October. More than likely these will be a mix of early mornings, afternoons and nights. Often in Summer you may get two mornings, two afternoons and two nights, then 2 days off. That really is a killer fatigue-wise.
From November to April expect 9 or less flying days per month. All the remainder will be standbys or training days. You will not be rostered any extra days off even though there is no need to have so many people on standby. You will only get the minimum 8 days off in 28. Don't know what could expect at MAN, LTN or BHX, but I would guess busier all year round at MAN and similar to LGW at BHX and LTN.
People are currently leaving to BA and Virgin as one might expect, but also to easyJet and Aer Lingus and many have applied to Middle Eastern carriers, Cathay and Thomsonfly.