TrustPilot and ForexPeaceArmy almost universally under-rate major brokers. This is because they don’t screen out the obvious incompetents who are are the cause of their own problems, but who blame it on their broker. Any big broker will have collected a huge number of these ‘problematic’ clients, so a poor rating is not of itself a reason to avoid a broker. Recurring themes in the complaints could raise a flag, however.
XM.co.uk (what I use) is regulated by the FCA, and by all the major regulators. They are a major player. I have used them, on and off, for years, without issue. Do take that with a grain of salt, though, as I only use their micro account (my version of demo trading, essentially) becasue I can get better spreads elsewhere for my main account.