maybe interesting for you mates. dont know if there are manufactors for handicapped bikers in england (but i think so).
hopefully no-one really needs this link - but i found it on our german gsx-r forum because a mate needed the adress to get a bike modified for a handicapped friend.
http://koeltgen.sitecall.de/bike/en/systeme.html?uid=1a8e0caff1d37d47b21ea5ede9e2ecc2
take care