Hi Rob, and welcome to SW!
Hermanni hermanni are charming tortoises and you're right in thinking they do not grow to a particularly large size. However, i think you'll find that finding true Hermanni hermanni is quite a problem in itself. Most breeders of this relatively uncommon sub-species of Hermans tend to keep the hatchlings to grow on and breed from themselves to preserve the line.
Any subspecies of Hermans ae a good beginner tort (if there is one!) as are Mediterranean Spur Thighs (Testudo graeca Ibera) I've been told that Hermans have managed to adapt the best to the UK climate.
I would also like to say well done for intending to buy from a UK breeder, rather than on-line or from a petshop.
Many of the on-line dealers and petshops tend to sell imported torts which although isn't a problem in itself...many people have lovely healthy imported torts...it has been reported that there is more of a chance of them having high worm loads, amongst other things.
Whether this is true or not I couldn't say, but I personnally wouldn't take the risk when there are plenty of UK breeders (depending on what you want) out there
If you have any more questions, please feel free to ask, there is always someone ready and willing to help
Good luck with your choice