Hey,
You have loads of options.
If you want to send the razor out, and fancy a day trip there is a guy called Gary Haywood in Nottingham.
The Straight Razor Specialist - Razor Sharpening, Honing, Whetstones and Strops. Razor Sharpening UK, Razor Honing UK, Razor Sharpening Nottingham, Razor Honing Nottingham
He runs a barbers shop and as well as offing honing services, He also sells a reasonable selection of gear. I'm sure if you book in and take your razor he will show you some tips as he fettles it. He also has some rocks called coticules that he is a specialist in, So if you do pop in have a good chat about those.
The 12K is a really versatile stone. If can do a fair bit more than just finish. One guy on here has even gone so far as chip removal and a 1 stone hone with one. If you decide on more rocks then there is a huge choice but it's only going to be worth doing this if you plan on expanding your razor collection and need a way to make ebay purchases shave ready. Once you have 5-6 razors sending them out can start to get expensive.
For decent chrome oxide look no further than a local model railway/airfix shop. There used to be a shop called antics in Sheffield but it has since closed, If you search for airfix stockists and give them a call they should be able to tell you over the phone if they stock the humbrol weathering powered. Humbrol av0005 chrome oxide green is pure pigment grade chrome oxide. I've been using it for years, If you mix it with a bit of light mineral oil into a paste it can be far easier to apply. The Humbrol container is good because it's got a secondary internal cap that also doubles as a mixing pot. In store it's normally £3-4.
https://www.amazon.co.uk/Humbrol-Wea...thering+powder
Personally I can use a crome oxide strop and regular plain stropping and keep a razor good for many months. Normally the only reason I get the hones out is because I have a free day and fancy a play! It's nice to have the rocks and be able to use them but it's not essential If your razor is already in good working order.