So Hyperoptic may be leasing IP addresses. They don't pay a one-off charge on those so it makes no sense to charge a one-off charge.
Either way they're charging what the market will bear. What they should charge is entirely subjective. What they may charge is down to them and their market.
If they are leasing that explains why an ongoing charge rather than a one-off.
Might be a 'dumb take' but regardless what you think they should be charging isn't what they've been charging for over a year and they are evidently comfortable with that pricing structure.
They could be charging way more for gigabit given the price their main competition, BT, charge, but don't.
Building better networks, not just faster ones.