In the nicest way possible, OP, if you haven't ridden for 10 years, you will be rusty and not able to balance your own weight as well as you used to, even if you know how to in theory. By all means ring the school, but be prepared to be told no. Half a stone isn't too much to lose, and getting a bit fitter may help you get more out of your lessons.
I do think 11st is very low for a weight limit- but some stables cater mainly to children and have mainly smaller horses and ponies. If that is what is safe for the stock they have and they are making a living from their business, then they don't have a duty to cater to heavier riders.
Most riding schools post their weight limits a little lower than what the horses can actually carry, as they know people will lie and round down- and they know that beginners do not usually carry their weight as well as an experienced rider.
As well, it can be harder to find weight carrying types that are really suitable for a beginner rider- they are out there, but they are pretty rare. Plus, they will often cost the school more to keep, in terms of feed costs etc. They can also be more destructive to fencing etc. Equally, many clients may find them uncomfortable to ride (e.g. too wide) so it is a balance for the school as to if it is commercially viable to keep such horses.
Heavy horses are not usually the answer to weight carrying- they are build to pull, not carry weight and many will have weak backs due to this. Also, they may struggle in the standard 40x20 arenas that many riding schools use for lessons.
True weight carriers are often things more like maxi-cobs, with short backs and a lot of bone or a true hunter type horse. However, good quality horses of this type that are happy to take a novice rider are in high demand, and the purchase price may put a riding school off.