It's always been x percent of population coverage rather than land coverage.
Apart from EE, because they have the ESN, they have to approach 100% 4G coverage of the UK landmass. Admittedly, ESN has a higher priority on cell sites so can grab a signal when non prioritised handsets have none.
But the kindest thing I can say about Evil Everywhere is that it's signal is "variable"

. Its mostly better than Three in most places and mostly better than Voda in the Home Counties north of London, usually on par or just below VMO2.
In places where it can get a strong (full 5 bar) signal that has genuine SA 5G, it's quick enough though, knocking on the door of 1Gb down, and around 150Mb up. The asymmetrical nature is making it less useful for a home broadband though, especially with many working from home, and/or the amount of cloud storage we all use.