You probably have most negotiating power in the last 30 days (or just approaching the last 30 days).
M125 + basic phone is showing as £29.99 for a new customer and £62 'standard' price from month 19 so set your expectations accordingly.
More relevant is what are the competitors in your area offering? That is the info you need if you are going to haggle with VM.
Try the advice from Andrew-G at message #5
https://community.virginmedia.com/t5/Forum-Archive/Why-oh-why-does-loyalty-get-punished/m-p/5226917#M218699
(an old post now but the advice is good)