It appears from a BBC news item that both BT and Virgin are being hit by a massive increase in Business rates ( 450%) to £743m.
My simple maths says that if this is all loaded on OR BB customers (20m) it equates to a £3.10 a month increase in wholesale cost.
A large amount of the revaluation is anecdotally due to the increase in fibre and street electronics but I cannot find evidence of the VOA methodology in the public domain.
I see no way BT could absorb such a huge increase in cost and Virgin will have the same issue.
If loaded purely onto fibre lines it would be £10.30 ( 6m lines) but this would slow the take up and thereby ROI on fibre so more likely to spread it over all lines.