For the UK, FCA regulation is what matters.
Check they're FCA regulated, check their regulatory record. These things you can do yourself, but the rest will be "impression" and forum recommendations of the type you're asking for.
I suggest Oanda. They're not perfect, but they're honest, ethical and well regulated. IMO they're as good as you can get for a small account which requires that type of broker (i.e. not a 6-figures account making a direct-access, comission-only brokerage a possibility).