Machine supply companies often sell hardened steel.....but this is a bad choice for knifemaking.
You are paying for HT when you buy pre hardened steel.
You are paying for it again when you grind/shape hardened steel by using more belts and sand paper. ( and you can't use files to shape things)
You may also pay for it by getting a lower quality finish and a blade with a damaged HT on the edge. ( unless you have good equipment and skills).
You would be far better off to send the blade(s) out for HT.... or get with another UK maker and have him do the HT on a shop visit.
The old saying is, "You pay it now or you pay it later...but either way you pay for it."
Filling out your profile location with something more specific than "UK" might get an offer of help. Edinburgh, Scotland and London, England are both UK.
Unless you are a crofter in the Outer Hebrides, it is likely there is a machine shop somewhere nearby that could do the HT. It is also likely there is another knifemaker somewhere in your region. Check out British Blades to se if you can get some local info and contacts.