www.wheelchair-travel.co.uk/index.html
I just met this lovely lady, Dawn Pollitt.
She and her hubby and a few friends run this company which hires out adapted vehicles for the solo (or those in small groups) UK traveller.
They mainly cover London and South, but do have vehicles i other parts of the UK Mainland, though the choice is more limited.
Anyway, If your thinking of hiring a car or van, then these are the nicest and most understanding people you could meet, and they will do their best to accomodate you.