I weigh a little over 200 and my 14.2 finely built Arab has no problem-- for example I worked with her yesterday w/t/c, collection, extensions, w-c transitions and 2'3" jumps. It was approx 1.5 hours of riding at a fairly demanding pace. I got off, tired and a little sweaty while she pranced around and came no where near breaking a sweat. I was fully prepared to hand walk to her to cool out, but clearly, not really necessary. This is not particularly surprising given I've been overweight for the past 7 years and she has shown no particular strain with our normal work. She also has no sign of particular back problems or other strain associated with carrying too much weight. Given my history, I'd think you'd have no problem with this horse. I have heard the UK is particularly touchy on this issue, and that the 20% model is strictly followed. I think it's more of a societal stereotype but that's my opinion of course.
However, I will make one point that no one else has brought up: If you intend on competing on this horse, judges (in the US) do look at whether you look to large on your horse AESTHETICALLY. This doesn't mean whether the horse CAN carry you, but whether you look like a good match in height, weight, etc. If the issue is larger in the UK, you might find placing in classes difficult depending on your actual look on the horse. It's not fair or nice, but it happens a lot over here.
Good luck to you!