On HFC they do offer 1 month (not sure about XGS-PON) but the monthly sub is so bad you would be better going for the 18, and then taking your chance of talking out of the exit fee, should the need arise.I may be able to order a Virgin package this year, via Nexfibre's FTTP network.
Is there any wiggle room on contract lengths, as I think an 18 month (rather than 12 month) contract could be a deal breaker.
One thing you could do e.g. if you want to leave is just say you moving to an area that has no VM coverage.