The two-year Foundation Programme is the bridge between you completing Medical School and progressing into your Specialty training. Although there are regional differences, you are generally based in one hospital trust for the duration of your training, with rotations in several specialties during this time.
You should choose a school which will offer you the Foundation Programme (FP) experience you would prefer, taking into account exposure to specialties you are considering as a career, as well as the support you will receive as a trainee. Of course, it is also important to consider where you want to live!
It is important to remember that Foundation Schools have different geographical footprints. Therefore, do your research before preferencing your chosen schools.
Also consider the quality of the training you will receive. The GMC National Survey assesses trainees' perceptions on a range of quality indicators, and you can access this here.
Applications to Foundation are ran via the UK Foundation Programme Office and more detail can be found here.