Not many places sell 'em so you'll have trouble finding something local, never mind rentable !
London Recumbents in Dulwich Park (and Battersea too) used to do recumbent hire - really for trying out in the park, but I'm not sure what they're a distributor for nowadays - used to be a Challenge distributor but I'm not sure they are any more (tho' they're still listed on the Challenge website)
http://www.londonrecumbents.com/recumbents.html
DTek up near Ely (Cambridgeshire) have a huge number of assorted 'bents to try:
http://www.littlethetford.org/?page_id=529
- typically a half-day in the local roads, tho' I think Kevin has some sort of buy-come-rent-if-you-return-it deal.
Westcountry Recumbents are now in Derbyshire
http://www.wrhpv.com/about_us/index.html
and are a Greenspeed dealer.
T'other one is Laidback bikes up in Edinburgh:
http://www.laid-back-bikes.co.uk/
Futurecycles in E Sussex has closed, Bikefix in central London sell Hpvelotechnik and Challenge - don't hire, but you can try one out in the local streets.
ICE recumbents - ie the manufacturer - are over in Falmouth.
HTH..