Looks like I left it a little late to cancel my X370 pre-order as I just got shipping notification for it. I don't expect that until Monday now. I still hope to make a start on testing with the B350 which is expected to arrive today.
I note the 1700 is currently in stock at Amazon UK, so only a day late. Higher models are still unavailable (excluding marketplace sellers).
A look at Scan where I'm getting the Asus X370 from, shows only the MSI B350 still available which seems like no one wants that one. I don't think that it is they're not getting stock of others, but that what little they get goes on fulfilling preorders so it doesn't actually show in stock.
At Novatech where I'm expecting my delivery today, they currently have 1700 and 1700x available, and a handful of the Asus B350 mobo.
At ebuyer, they also have 1700 and 1700x, but the layout of their website means I can't see at a glance mobo availability without clicking each one.
Not a comprehensive list of UK sellers by any means, but just my main go to sellers. In short, lower Ryzen CPU availability seems to be here now, but mobos are still in short supply.