I would say in England £20 gets you roughly 20gb of good quality internet, but there are lots of plans that will sell you way more for way less with poor coverage and throttling.

The post implies that they found a rate which was $2 for 20gb in the U.K. in their cheapest tarrif - which I assume was some sort of ‘unlimited capped at 200gb for £15’ style deal which would be heavily throtttled.