I had a friend at uni who joined the Army equiv of the UAS, can't remember what it was called, in order to make friends and socialise. Decided after a couple of weeks that it wasn't for them and resigned. About 5 years later they received a call up letter to go to the gulf. Apparently no matter how long she served she was down as a reserve for a minimum of ten years from resignation.
I don't know if it's simlar for the TA, but I didn't think the UAS equiv would be regarded as regular forces, I would have thought that it was more akin to the TA.