For all new recruits including cadets, the bases on offer depend on the local demand. Preferences may be expressed and noted, and of course it is always going to be satisfying to be able to offer somebody the choice they want, but the job location is what it is. For most airlines the majority of vacancies are likely to be either at a main base, a new base, or a base that is expanding.
Of the cadets we have taken on over the last few years the placement demographic has been about a third each to the South, midlands, and Northern bases, with a slight bias towards the midlands. There can be opportunities for people to move base as vacancies arise, and there are instances of mutual swapping (with the companies approval). The majority of people (with us) do seem to find it works out eventually. Of course the very nature of this job may mean you need to consider making a move to an area that is likely to offer you better prospects within your chosen career.
The intake course I followed about 2 years ago, all started their training 15 months prior to that (Autumn 2010). None were pre-selected by the airline prior to the completion of their integrated training courses. They all now have their ATPL (unfrozen licences). They all have around 1500 hours on modern jet aircraft. They can look forward to meeting the requirements for command qualification in around 3 years from now. They all gross around £60K per annum plus benefits.