And since no one mentioned it on the magazine portion...
If it's a true Saiga, it will not have a bullet guide as part of the gun, and will need Saiga mags.
If it has been converted (as much as it can be), if UK-imported rifles aren't fed the same as US ones, or it otherwise does have a bullet guide for whatever reason, it will use AK mags. You can then modify any Saiga mags to function.
If it does not have a guide (and if AK mags are cheaper in the UK than Saiga mags), consider doing about twenty minutes of smithing to use AK mags, for the ease of finding them.