There are a lot of cycle tracks (mostly old railway lines) in the UK that have bike rental, however very few of them have adult tricycle or recumbent tricycle.
Does anyone have the list of where you can rent an adult tricycle in the UK?
Where abouts in the UK are you? You might want to check out the following places:
Hope you get sorted.
Battersea Park in London has these for rental for both adults/kids. It's only for riding around the park, but this is probably a safer place to experiment on than London's roads...