Reddit mentions of BOSS RC-1 Loop Station

I'd say go for the Boss RC-1.

It is a little over £65, but it is worth the money. The boss pedals are top notch quality and once you get the hang of how to control it (takes a little getting used to tbh) you will love it.

I'm a huge fan of both the Privia and Yamaha P-series. I personally like the feel of the Yamaha a slight bit better, but it's not a really big deal and to a certain degree the key action is a bit of a personal taste thing, particularly for digitals.

I would also say that for your case in particular (8 year-old, autistic, non-verbal), it's probably worth going for the Privia 300 series over the 100 series simply for all of the extra sounds and options that might allow him to explore more ideas more fully. Things like recording into the digital piano memory and being able to play over the top of simple loops might very satisfying for him and create a larger creative potential.

You also might be able to find older editions of the 300 series (rather than the most recent 360) and get most of the same features at a lower price.

If you want to save money though and aren't sure how valuable or accessible any of that would be for him though, I'd definitely recommend the Yamaha P45. Some features can be added on later. Things like looper pedal could add some creative functionality. There are also other ways to get more sounds, but they can be be pretty complicated compared to having the sounds on board (running through PC software and such).