UAS is exactly what it says on the tin, a University Air Squadron, go to uni and join the UAS. May be wrong, but I believe they are part of the RAF Volunteer Reserve, and as such not a guaranteed route in, certainly will look good on your application though.
If you don't want to fly military, but rather just want to use it as a step to your commercial, I wouldn't advise it for a number of reasons.
First, you still have to pay for the licence at the end, with limited aid. Not sure if aircrew/commissioned get any extra, but after 4 years I was entitled to 3 grants (no more than 1 per financial year) of £1000, and after 8 this rises to £2000 a time.
Second, there is quite a large return of service I believe for pilots (around 8/9 years). After this period you are still in a similar position to anyone else looking for their first commercial job, albeit with more experience and less debt.
Third, if you don't have your heart set on the military then the recruiters will more than likely see through you in the series of interviews you will have to pass.
Fourth, I know a lot of people in the flying training system are waiting for long periods between training courses, if the military's not really where you want to be, I can imagine this would be extremely frustrating.
Sure going through the military has its advantages, but I would leave it to those who want a career with the military.
Also, should probably be asking this on the military aircrew forum.