As others have said Wales, peak District, lake District. Just those places alone have enough for two weeks solid riding let alone what Scotland has to offer.
Do you have a route planned out? If you know where in the UK you'll be people can advise on where to ride. No point recommending golfie if you're down in Cornwall.
There's dyfi, revs (actually that might be closed now?) Antur stiniog and one giant leap Llangollen that I can think of off the top of my head that are uplift.
Bpw though has a massive amount of trails compared to any of those and the others are more downhill focused than flow.
Almost all the top brands and their mid range to top spec ebikes are carbon and have been for some years now. If carbon fibre was an issue they just wouldn't be getting the sales they have.
I was the same when getting a new e-bike, wanted aluminium for piece of mind but you're very limited in...
Just a thought have you tried bringing this up on their twitter/FB social media page? Often companies aren't keen on the bad publicity and make more of an effort.
It doesn't slow you down more, you're just expecting the acceleration from the motor assist which is no longer there, you're cranking with leg power only but your expecting the huge torque burst from the motor. You will go faster, just not like you're expecting to.
The issue isn't that they don't want them competing, that's perfectly reasonable, it's the patronising and condescending excuses for not allowing it. 'lack of skill' etc. Which is nothing to do with the bike, that's the rider.