Tom Anderson wrote:

Indeed. Victoria just about manages gates, but has a different design
that lends itself to them. Similarly Liverpool Street has natural places.
But where would one put the fare gates in the concourse/platforms at
Waterloo? The entrances to most of the platforms are too narrow.

Could there not be a gateline a few metres back from the platforms?

It's probably where it would have to be, but if it's going to work at all
then the concourse needs some rearrangement to move the shops, monitors,
steps to the Underground and everything! Waterloo gets very crowded at peak
hours and removing several metres near the platforms will only exaserbate
the congestion.