Ok more or less I understand, so you don't really know when the company is going to give you your preference base right? I mean there is no seniority..
Correct, no seniority, however the base transfer list is organised on first come first served, i.e., it's done in order that you join the list. As has been said, ALC is usually undercrewed so my guess is that you would likely get that base right from the start.
As for how much you make nett, it depends on which base you are at, as your sector pay and duty pay makes a difference. At ALC you will earn less than most other bases as you fly less, it also depends on how much you choose to pay into the pension, and also on the Spanish tax rate vs UK tax.