What if they put the prices up to £100/month if they realised they were the only provider
If they did that, someone else would eventually appear to sell the service for less. This is the part where competition kicks in. In the recent announcements in Southwark (SE16), both Gigaclear and Hyperoptic will now start a large, mostly overlapping project there. Which indicates there is interest to compete even with the current £35/150Mbps pricing level.
If Hyperoptic singled out just your building to charge £100/month, they would need to change their advertising, starting from their web pages. If everyone pays £35, they can advertise that. If it is a different price based on where you live, they would need to change to "prices starting from .... please contact us to get an individual quote", which does not quite sound the same. A lot of hassle for being able to gouge a few extra quid from those who do not have any choice. They would also need to constantly update their "rip off" -address database and remove £100 pricing as soon as competition appears. Managing this could well cost more than it ever generates in revenue.
And last, USO is not
a proposal to provide cheap and fast internet for everyone. It is only focusing on the "fast" aspect (which of course can be debated but in another discussion). If you have fast internet available but you are not happy about its cost, USO is not the mechanism to fix this for you, and it was never meant to be.