The short answer is... not very prevalent in UK bats at all, stickle, and you are at no risk whatsoever unless one bites you. 
The longer answer is here, and explains all the figures (and will reassure rather than scare you!) ... www.bats.org.uk/pages/-bats_and_rabies-1099.html
Something like 75% of all bat sightings are of pipistrelles, and EBLV (the rabies-family virus that some bats can carry) has never been found in a pipistrelle in the UK, nor in long-eared bats, which are the second most likely ones to come across. We have 18 types of bat in the UK, and the only bats that have been found in the UK carrying the live virus are Daubentons bats, which you are fairly unlikely to come into contact with, as they hunt over water (having said that, I found one huddled on a windowsill on Burford high street, so you never know!). The testing scheme found 14 bats, out of over 15,000 tested) that were carrying the virus, all Daubentons, and a further 3 carrying antibodies that indicated past exposure - those were a Serotine and two Natterers bats.
For the virus to be transmitted, you would have to be bitten, and the only way that would happen is if you're handling a bat. For that reason, people are advised to always wear gloves or use a cloth when handling a bat. All bat workers have to have rabies vaccinations, since a Scottish bat worker contracted rabies and died a few years ago. He had been working in Europe, though, so is far more likely to have been infected there, where it's more prevalent. The risk to you from the bats foraging over your garden is non-existent.