Sorry to be late answering your post, I saw both & opted to try & answer most of your questions in this one.
As for the cage the alternative you mentioned in the other post the alsask is ok, it's a good size but it is very tall for a Russian ham as they are not good at climbing, they can climb up but not down so there is a risk of falls & the cage needs to be set up carefully with shelves or ledges to avoid that.
Zooplus do sell the hamster heaven cage which is also a good size but not too tall, tall enough for a wheel (8") with substrate or a house underneath though. It is best used without the penthouse & tubes & the holes can just be blocked off with the bits provided.
I've have kept Russian hams in that cage & they do fine.
You won't find a suitable food in the uk but you could feed either the bunny dwarf hamster dream expert from Zooplus which is ok or a better food from
Rodipet a young ham will need a bit of extra protein added to either mix but that can easily be done with a few mealworms, tiny pieces of dried chicken (cat treats that are just pure dry chicken are fine) & mine all love a bit of plain tofu too.
Fresh veg can be given daily but should be introduced slowly at first, I would avoid fruit or sweet veg, if you need a more specific list just ask.
I use Kaytee clean & cosy (just the plain white) which is available from various places so quite easy to get hold of, it's the best I've used.
You don't need to clean the cage much, mostly spot cleaning pee than just partial cleans when needed, you never clean the whole cage at once & try to leave the nest & hoard alone as much as possible unless peed on.
Chinchilla sand is fine for the sand bath.
A lot of info on the forum is applicable to all species but not everything, it's a lot to answer in one post but if you have any more questions or anything isn't clear just ask.