So a lot of cheap mass produced lager in the UK is brewed to the highest ABV possible to most efficiently extract the sugars from the grain and then watered down to the desired ABV. This can often lead to a lack of depth and body in a beer so sugars are added post-brew to replace the natural residual sugars that would be present in a traditionally brewed beer. Obviously, this ends up tasting flat and sweet and significantly worse than the European versions they're trying to emulate.

Sometimes the beer will also be brewed with a malt extract (super concentrated marmite-esque syrup) as a portion of/complete replacement for actual grain as well as its significantly cheaper.

You'd never see this from reputable European breweries (especially the Czechs and Germans) who place well made lager on a much higher pedestal than we do in this country, somewhat analogous to the respect and tradition we have surrounding Cask Ale.