I don't know the details of the UK contract, but for the upgrade, Luton or Gatwick are in general the most likely assigned bases. People from all the other base are sent there for the command. Always in demand of pilots there, and big turnover, with the european guys usually desperate to go back to the continent.
The command process is very long in easyJet, with the first steps starting six month after the employment start, assuming you have the required hours and your performance is up to standard. Some other stages can be completed only after one year and two sim events in the company.
I would not plan on less than 1,5 years from date of joining to the start of the line training of the command course, and that's already pushing it.