Setting aside the day to day practicalities of off grid living (always watching energy use, no electric heat or cooking, the possibility of running out), the main issue is that this is the middle of the city. A block of apartments might be 6 storeys tall - a 50m2 flat would therefore only have 8.3m2 of rooftop - there's simply not nearly enough rooftop area to generate the power the building needs.

Solar wouldn't even be particularly useful for peak compensation, as generally the UK peaks fall on winter evenings, while solar only makes useful power in summer.

A better solution would be just the batteries. If these buildings had enough battery storage to compensate for usage during the few peak usage hours per day, the effect of new buildings on the grid would be negligible (as the grid is basically underutilised for most of the day). Then the solar could be built much more cost effectively in a field out of the city.

A single home with all of its roof covered in solar panels is barely self-sufficient, even during the summer half-year. It can be, but it depends on usage.

Because such a system wouldn't work in the winter. New homes in West London would almost certainly be in the form of multi story apartment building; so there wouldn't be enough free surface area to install solar panels to collect enough energy even if there was enough sunlight.

Solid enough to not have power failures after days of rainy weather? Maybe a more practical alternative is a generator.

Isn't london famous for its rainfall? kinda feel like solar wouldn't really work so well there.

The whole UK is on the grid, even if you have solar generation, you sell the surplus back to the grid