Why not just locate the AP in BR01 rather than the loft space? It removes a layer of ceiling between it and the clients. Also if you're bothering to do this then buy actual decent AP's, not ADSL routers - a 2.4Ghz Unifi AP or AP LR kit will be inexpensive on the bay of thieves and is easily feeling mounted (LED can be turned off via the web interface). Why 2.4Ghz? Because with thicker walls 2.4Ghz achieves better penetration than 5Ghz and as you state ADSL router, you'll be WAN limited anyway even if you go VDSL.
Powerline can be OK, but it does vary from property to property, it's always going to be inferior to having a 5e run. In general stick to a master socket and not a spur, an extension plugged into one of the dual sockets is fine, plugging the PL kit into an extension is not fine, same with surge protection - don't do it. Try and avoid plugging it directly next to something that is likely to cause noise/spikes like a fridge/freezer, that's not a great idea.