Aside from the RAF Air Cadets and VGS there are civilian clubs who do have paid instructors. However there are not a huge amount of gliding clubs in the UK compared to the rest of the EU and from my experience the clubs only tend to employ a handful (if any) instructors. The rest of the instruction being done voluntarily by other club members with instructor ratings. That is not to say that these positions don't come up from time to time. The best place to look is on the classifieds section on gliderpilot.net .
To instruct in the UK you need either a BGA BI/AssCat/FullCat rating. or now you can instruct with EASA SPL or LAPL/S with an FI rating. To instruct on a motorglider you will obviously need the approprite rating also tagged on to your licence.