Given how much decent whetstones cost, £6 is not so much a bargain as a red flag IMO. It'll be relatively soft, and very easy to dig your knife edge into if you over-angle the blade or press too hard. Also, it'll probably take much longer than a more expensive stone to put an edge on harder steels.
That said, at £6 you can always buy one to practise on, if you've not used one before. Practice on one of your crappier knives, not your favourite Japanese knife, but do be aware that the results you get on a cheap Western knife won't be the same. I actually struggle to get a decent edge on my Wusthof with the same whetstones that put a super-sharp edge on my Japanese knives, but that might just be me not adjusting my technique properly.
I've never tried to use a whetstone with an angle guide still attached (always figured it would scratch the stone), but yeah, 20 degrees is generally going to be a bit too much for Japanese knives. That said, it's a complex rabbit hole to go down, and there isn't really a universally "right" angle to sharpen at. But sharpening at a right angle would definitely be wrong
400 grit is a perfectly reasonable grit for re-sharpening a blade if it's gotten really blunt, or working out small chips. To be honest, for most of us home cooks you can put a more than adequate edge on a knife with a 400 grit stone. All the silly paper slicing tests I do at home are just hobbyist b******t really, I'm not doing anything in my kitchen that
needs that sharp an edge