Ironically London is one of the worst served places. Outside of hyperoptic noone is focussing on flats (MDUs), and inner London has a lot more apartment buildings than elsewhere.

I think partly because it's congested and expensive and awkward to install stuff. I'm in the center with 8 Mbps adsl over copper lines. One of my friends got 1GB fiber to his office put in but had to pay ~£3k for them in install the line.

It's because you're in London. I have family in rural Cumbria with full fibre, yet none of the flats I've rented in London have availability.

There are holes.

Sometimes they are caused by property owners not allowing entry into a block.

Eventually those properties will have to be dealt with, but a legal change may be necessary.