Talland is the only place I know of that offer that level of school master - I'm not sure where else I could go. Suggestions welcome!
I've had a couple of lessons at Talland, they are school horses so they are pretty safe types, but the second one I rode had some super tricks up his sleeve when he got going although it took a fair amount to get him forward. I think if you pay extra you can request one of the "elite" horses which I presume are a bit more forward thinking from the off. I definitely enjoyed it though.
There is a lady in Congleton (near somerford park) who offers school master lessons - can't think of her name off the top of my head - Michelle Walker maybe - but she is on the BD group on facebook so if you put something up I'm sure she'd respond. It's still a trek of course but nearer than Talland. I haven't been to her as at the time I was looking I came across my super cob and now have a regular ride so all my pennies are going to training with him, although I haven't ruled out booking a lesson just to go and say I've ridden a line of tempis!
Bit further south Ingestre would do something I am sure.
Possibly nearer home for you, I'd consider at least speaking to Yorkshire Riding Centre - I don't know about now but they used to run exams up to at least stage 4 and have students so I would expect they'd be able to give school master lessons.
You might also be surprised by a local decent riding school - where I used to work one we had a couple of advanced medium horses used for higher level lessons occasionally, so it is worth an ask.
On the getting to Olympia thing I am going on Sunday on the train from Manchester. Obviously it wouldn't work for the evening performances but it only takes a couple of hours to get down - I guess if you are committed to the dressage nights that would be a bit different but I am just super excited for the "pantomime" lol