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UK has switches on all the sockets. In states we don't, so all our sockets are left "always on". Might be situations switch off is safer, but here it's out of the way, and I'd think that's extremely rare where it's safer anyways. Sounds fine to me.
I don't think it will matter whether you leave it on or not. The EVSE will draw a little current, but not a lot. The only thing I would avoid is turning it off while the car is actually charging. Switching off maximum current can't be good for the switch.
Note: in this section you're going to get mostly USA responses. There's a UK specific forum where you will get more UK readers. I would also suggest amending your location in your profile to specify the UK.