From my recent thoughts about moving providers it seems the only UK e-sim providers are BT Business (not BT Mobile), EE, Vodaphone and O2. Your phone data usage against cost is probably the main decider.
BT Business is not applicable for most users.
EE is expensive as you need you basic phone contract about £20+pm and a watch contract about £7pm.
O2 seems reasonable as you can get a basic phone contract from £5pm plus £5pm on the same number for a watch contract.
Vodaphone is from about £12pm for the basic phone plus £7pm for the watch.
For now until I decide on new watch 4g provider, when I don't want to take my S21 Ultra out with me (it's a bit big to be in my pocket on a golf course), I take my old small phone and use it a mobile WiFI hotspot, connect my Watch 3 to it by WiFi and get all the connectivity between my S21 at home and the Watch 3.