I did this at one point in a house I owned in CA, in my kitchen, with PLAY:1s. It was easiest to work with my electrician and the actual mounts themselves to determine a good location. I do remember making or buying shorter power leads for those locations, so I didn’t have wrapped or dangling cords to deal with. If you haven’t yet, I’d encourage a Google search or two. I seem to recall that within months of installing, someone in the US came out with mounts that literally included a power plug. I don’t think I ever searched for a UK version, but it’s possible. 

A box like this would allow you to slip any extra cord into the brush slot. Finding one to your electrical code should be possible.

https://www.amazon.com/Concealer-Connection-Management-HT2102WHV1-Hardwired/dp/B003JUBMCI/ref=sr_1_2

Which reminds me. I’ve been using the Sonos Sanus mount/stand for years. Just at ground level, which I wasn’t thinking about at height. It isn’t tillable, though. 
 

This may be what I was originally looking at. The name Midlite rings a faint bell. It would have gotten rid of the wall plug, but I think it was in beta back when I was looking at it. No personal experience on that device. 

For wall mounts where do you place the power outlet and how do you hide the power cord?

Isn’t that covered in the posts above? 

An alternative is to run the power cable in some trunking.