Having a PPL is not a bar to UAS membership. As UASs are now overtly recruiting machines for the RAF, rather than for instilling 'air-mindedness' in under-graduates, as they once were, you will need to have an interest in joining the RAF as a career. For greater than 5 flying hours per year, I believe you would need to be fit for, and aiming for, employment as aircrew within the RAF. This is somewhat different from when I was a UAS CFI, so perhaps those with more recent knowledge will confirm or correct my statements.
In any case, we always found that those with a PPL would have an advantage in the early stages of training, but would sometimes need extra tuition for the more advanced elements of the course. This was for three reasons: one was that it took some time to eliminate bad habits from poorly-taught PPL students; another was that some PPLs were already mindset into civilian-style flying and didn't take well to the more disciplined but extrovert military style; and the third was that, whatever your previous experience, some are simply more talented than others!
A last word of warning: do not attempt to use a UAS as a cheap way of adding hours en-route to an ATPL. You will probably be found out and ejected in disgrace - and bad raps have a habit of following you around! Best of luck in your application to join.