I'm with an altnet for an gigabit symmetrical service. Only £40/month. Great value!
Not sure why OR don't offer this.
A number of reasons.
OpenReach FTTP is GPON, which is asymmetric.
It's 1.2Gb/s upstream split up to 32 ways.
OpenReach guarantee the upstream speed they sell to ISP's.
Allowing a single customer to take 1Gb/s of the 1.2Gb/s available to the up to 32 properties doesn't fit with that guarantee.
CityFibre is also GPON but they don't guarantee the upstream in the same way.
OpenReach also have very large leased line revenues which cheap symmetrical gigabit would likely eat in to.
They are also highly regulated unlike your Alt-Net.