Where should I (not) place POE ethernet outlets?
Well, I will strongly suggest the middle of ceilings, because that's where you are going to want you POE wireless access points to be wired in for best performance, particularly in a solid brick walled house (very hard to get fast WiFi through brick walls at all.)
I will also suggest Cat6A if you have dreams of 10GB. While 6 is nominally good to 55m, the price differential is small, and 6A might offer some slight hope for whatever comes next for copper wires when 10GB is too slow.
But that also leads to my stock firm recommendation for wiring any building for network - use conduit, the cables become obsolete MUCH faster than the power wiring or the house. Being able to pull out your 6, 6a (or 5e if it's much cheaper for you now, though it's usually not that much cheaper) and replace it when you need something faster with whatever is in vogue then will pay off in the long run.
I don't know if the UK has any codes specifically regarding POE ethernet - in the USA as power-limited low voltage it has very little code affecting it (primarily the communications cable section of code, which does not have any notable location restrictions that I can think of at present.) Presumably as an international standard it might have been designed to avoid issues in most countries, but some may have more restrictive rules than is the norm for that application.