Just out of curiosity, I just took a look at the tyres on all three of my trailers and they're all 'R' rated (low speed) BUT I've never experienced a problem towing at 70-80mph (completely illegal) or even on several occasions at 100mph plus (really, really, completely utterly illegal)

I've never been aware of high speed rated tyres for trailers. In any case I think 'R' rating for tyres is 'up to 106mph'