Right now they are rideable. I was over there (in the car) today; further East (Berkshire) is a lot snowier
But we don't know what'll happen in the next 2 days. If it were me I'd start (the bit from Cardiff to the English side of the Bridge will be rideable) and then see how it pans out. If the lanes are grotty, go through Sodding Chipbury and stick to the B-roads. If the B-roads are blocked, go back via Chepstow garden centre, have a cuppa and enjoy getting 100 in for day
We rode the Cotswold Corker a couple of years ago in a similar situation; the lanes had been impassible the day before and only 65 out of 265 started. It was a great day and we got round fine.