First of all, neither slate nor granite should ever be used as baking stones. I sincerely hope that the store selling these isn't marketing them in this fashion, especially granite. Slate is very soft and flaky and will most like just crack in the first couple uses, but granite is mostly silica and can shatter like glass with extreme temperature changes.
Unglazed clay tiles are an option, but they won't be conductive enough to work well in a 550 deg. oven. Also, they most likely won't be thermally strong enough to handle the kind of oven tricks you'll need to incorporate in order to hit Neapolitan temps.
Here are a few options:
1. Forget about Neapolitan, for now, buy the unglazed clay tiles and mod your oven so that it bakes at 650-700 F. You should be able to hit 650 with the frozen towel trick (do a search).
2. Find a ceramic supplier and purchase a 1" thick cordierite kiln shelf. This will most likely work well for NY in a 550 deg. oven, and, when the time comes, will stand up to higher temps for Neapolitan.
3. Find a source for soapstone. It's extremely difficult to find in the UK, but if you do some digging, you might find a source. It most likely won't be cheap, though. The advantage to soapstone is that it's more conductive than cordierite and is guaranteed to bake a 4 minute pizza (the sweet spot for NY style) in a 525-550 oven. Another advantage of soapstone is that, when the time comes to mod your oven to hit Neapolitan temps, you can shoot for a slightly lower temp (750 instead of 850) and expose your oven to a little less duress (high temps/cleaning cycles can be hard on ovens).
I would try these guys in Glascow
M. Teixeira Soapstone UK Limited
p: + 44 790 860 0138
M. Teixeira is a pretty respectable Brazilian soapstone importer here in the U.S., so hopefully their Glascow office will be knowledgeable. See if they'd ship you a remnant (if your oven can fit a 17" x 17" x 1.25" slab, that's ideal for 16" NY style pies). The remnant most likely will be expensive, but the shipping will be exorbitant. You're looking for a classic high talc Brazilian soapstone. The good thing about Teixeira is that they only sell soapstone and should know what you're asking for. A lot of granite and marble places may tell you they sell soapstone, but when you get there, it's something else such as serpentine.
The UK has plenty of ceramic suppliers, and, if you do some comparison shopping, you should be able to find a kiln shelf locally for very little money. Just make sure it's 1" thick. I would probably recommend getting up to speed with cordierite and then keeping your eye out for soapstone.