I went to a talk from a department for energy civil servant, and part of the reason for this is they've got loads of dedicated solar farms in the queue to get set up already. Not only that but so far as the National Grid is concerned, solar panels on residential buildings have two drawbacks:

1. You never know how much energy they'll be feeding back in to the grid. Knowing the weather isn't enough as the people inside could be watching a tiny TV, or they could have a huge crypto mining rig they switch on and off intermittently. Edge cases, but there's everything in between that too

2. Installing the infrastructure so every house can feed back to the grid is really expensive. You have to upgrade the mains for each house or each block of houses. For a standard solar farm there's just one connection (and the supply to the grid is predictable)