In the medium term (without access to the figures) I would expect that your chances would be good. Keep in mind that every promotion in the base means a new FO slot.
Depending on your experience you might want to retain the flexibility to take a promotion at another base also.
There are plenty of guys being promoted at LTN at present so worst case might mean that you have to drive down there for a while.
If you are in it for the long term none of this should bother you too much.
As far as other bases go, it depends on commercial opportunity but I believe that Paris and LGW will expand as rapidly as slots allow.
Good luck with your decision, personally I don't think there is any!
NG