If you are flying using a UK Part FCL licence you need a sailplane towing qualification. If you have a UK licence other than UK Part FCL there is no such rating.
If you are towing gliders in the UK, other than in a military environment, it will almost certainy be at a BGA club. You need talk to the chief tug pilot at the club about what they require. There will be minimum amounts of power flying required (to meet insurance requirements unless you are towing with your own aircraft) and you will need to be trained to an equivalent standard to that of the Glider Towing qualification. The Sailplane Towing rating training is based very much on the training that gliding clubs in the UK gave their pilots before the rating was created so it is very relevant. Unless a gliding club is desparate for tug pilots they will typically require some gliding experience. The reason is twofold. You become aware of the way that gliding operates - there are many differences from power flying. You need to be able to 'read' the sky to take gliders to somewhere they can soar; it's not just throw them up - it is giving them value for money.