Until April next year there are two ways to get it.
1. Find an FI(A) or CRI or CRI(R) at an approved flying school who has the rating and can teach it. In practice that is almost impossible to achieve.
2. Get a gliding club to teach you and complete the training by say end February 2018 so you have time to apply to the CAA and get the rating added to your licence before 8th April 2018. This route is to obtain the qualification under BGA rules and then convert it to an EASA rating under the licence conversion rules published in CAP 804. Most gliding clubs use a very similar theoretical and flying syllabus to that published by EASA.
After 8th April it will have to be an FI(A), CRI or CRI(R), however, whereas the BGA is not for this purpose an Approved Training Organisation it is expected to become a Declared Training Organisation at some point so it is likely - once that happens - for it to be able to do further training.
The theoretical syllabus takes about half a day of briefing at most. The flying training is of the region of ten tows plus some towing in a glider if not already a glider pilot. Details on EASA web site and also the BGA web site.
A CRI(R) is a CRI restricted to teaching aerotowing and not able to perform the other roles that A CRI can do.
There are lists of gliding clubs in your area on the BGA web site.
A better place to ask this question is
Glider Pilot Network > uk.rec.aviation.soaring in the u.r.a.s forum. This is the main forum for gliding in the UK. The r.a.s forum is more American and the u.r.a.s.b forum is best avoided unless you want to receive insults and read bad jokes.