The warm/flat thing is a peculiar (often American) stereotype and it's a bit of a misnomer - I'm gonna try to dispel it a bit because it's quite an interesting topic.
The beers you're talking about are served cool (though not ice cold as you're actually meant to taste something) and are generally quite good - they're made in smaller breweries often serving local communities and are usually served in pubs that tend to take pride in what they're serving[0] - cleaning the lines between each keg, monitoring temperatures in cellars, discarding kegs that are a bit off rather than serving them. "Flat" implies something was infused with carbon dioxide to become fizzy but then left out accidentally to go stale, whereas these are purposefully only as fizzy as the brewer intended. Sometimes there's a light carbonation, sometimes not.
The beers you want to avoid are those that are mass-produced and what I'd describe as "lager adjacent" - Carling, Fosters, Stella, Kronenbourg and whatever US stuff makes it over. They're served everywhere and are, in my opinion, quite underwhelming if you're used to proper lager-type beer (idk what the preferred English nomenclature is, here it's ležák).
I definitely prefer Czech beer but UK ales are something I do enjoy.
[0] - let's not mention Wetherspoons