The base where you'll do line training is rarely where you'll be based afterwards.
The usual practice is to move base shortly after completing line training and stay at the second base until command comes - they use command as the first carrot to keep you in line. Then, even if a space is available at your preferred or existing base, you WILL be transferred somewhere remote, away from family, on reduced pay while having the expense of your family home in one location and digs at your base (they justify the reduced pay and low European scales on the basis of living costs being lower there, ignoring your family costs at home as if all pilots are single). This is done typically for another two years, seemingly in order to keep you in line and subservient to get the base you want. I have not seen personal circumstances such as ill family or young children have any effect on the policy.
There are promises to make basing transfers faster and more transparent. I don't believe a word of it. Basing and leave are two of the biggest reasons for so many hundreds of pilots leaving every year, and while it is a business not a club, and they need to put people where the slots exist, there is a lot of apparently deliberate misallocation of basing.