Wow - I've never known Virgin do this!!
I would point out to them that not offering you the ticket you paid for based on your mobility seems discriminatory and that you've traveled with VS before with the same issues without problem.
on many LGW routes the second half of UC known as Zone B is set aside for limited mobility PE pax. However on Carribean routes they seem to operate both zones as UC.
I once flew to Barbados and was upgraded to UC because of this.
I would certainly email someone else at virgin with your concerns and as alan says maybe pop into a CAB etc for advise also you might want to check out
this siteGiven the short notice I'd also check travel insurance etc in case you need to make alternate arrangements with other carrier