Lol, wrong side of the country, not a huge fan of London, too many arseholes and too many foreigners.
I'd have thought for clones or just a pot of it you'd want to find a very good garden center, which most these days do not seem to be, or find one of these small nursery type places out in the hills where they're just pulling and potting random herbs and shrubs an whatnot that they just have growing on their land.
I believe that sage is fairly easy to grow, lots of sunshine being the key thing due to it's Mediterranean origin.