I've never seen Thai Basil in any of the Asian supermarkets in Glasgow but I have found it in Asda of all places! It's in the fruit and veg aisle beside the herbs, you might just find an empty Thai Basil box though, I only find it about 50% of the time.
I don't know if all Asdas stock it but I've seen it in the Hamilton one (off the M74). They also sell fish sauce and Tao Kae Noi (crispy seaweed) and a few other handy bit and pieces in the Asian aisle.
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