AFAIK, the only main time you can take your seatbelt off driving forward is if your quite far into a pregnancy, but then you might be a bit stupid driving anyway.
Nothing about reversing on the seatbelt page:http://www.direct.gov.uk/en/Motoring/OwningAVehicle/AdviceOnKeepingYourVehicle/DG_4022064
Thinkroadsafety.gov.uk said:Exemptions from seat belt wearing
There is a specific exemption from seat belt wearing on medical grounds. There are some other exemptions, for example when reversing.
Nor is there anything about seatbelts on the reversing page
...
But there is something on the governments Think! Road safety website
http://www.thinkroadsafety.gov.uk/advice/seatbelts.htm
Why would you need to? I've never found a seatbelt to be uncomfortable whether reversing or otherwise. In fact it feels very weird if I don't wear one.
Well, i reverse with my eyes closed anyway