The best advice I can give is to head to HotUKDeals.com and check out the listing's there. People will post up when pre-orders open in places, but like last year it's selling pretty fast. You can set up alerts on there (or via the app) or even just keep a tab open on the Gaming category for new items and just refresh it every so often. I managed to grab one for a friend from the Nintendo UK Store as birthday present today from doing all three! Lol
As for retropie, I agree it's cheaper and more expandable, and I actually had a Pi zero and a Pi 3 before my NES mini, but I think I just prefer the simplicity of the NES mini. I've had to reinstall retropie a handful of times if something is buggy and it can be a long process with a lot of roms involved. I like it for PS1 games, but for a quick game of Zelda, Castlevania or Metroid etc, I definitely prefer the NES mini.