I think it will come down to around £100 a month inc vat for top tier 1Gbps / 1Gbp from 2026 onwards from BTw to ISPs.
On what basis do you think that? Why would they want to reduce the price? They will get no more customers at an £83+VAT price point than a £100+VAT price point, so by reducing the price they would only reduce their profit from the business customers who take it.
Remember that OR could have reduced the 1000/220 product which has been around for years, at any time they wanted to. They haven't done so, not even after announcing 1G/1G.
They are simply not interested in competing in this niche space for consumers. The altnets *are* going to take some customers, or else they will fold - and OFCOM won't allow that. Therefore, let them take all the high bandwidth uploaders and deal with issues around PON contention. Openreach keeps all the rest, and the vast majority (and I mean vast) don't give a hoot about upload speeds.
You have to remember that people like us, who talk about broadband all day, are not representative of the population as a whole. Most people just see it as a tool to watch TV and do their shopping. Those are the customers Openreach want to capture.