I think Asda worked out best for me too when I had an A7 because they were running voucher codes etc. Even less than the likes of Blackcircles/Tyre shopper
I had stuck with Conti's after a very bad experience with Dunlops shredding (known issue with the A7 and Dunlops!) and they held out well before I chopped it in last year.
I do not miss shelling out for 21" tyres although 20's are not that far off
Im running P Zero's on my summer rims (20") and Goodyear Ultragrip + on my winter rims (19")
I have a very friendly independent tyre dealer close to me, who is happy to have any online purchases shipped direct to them - regardless of who the supplier is. They also store whatever rims Im not using in their warehouse for £30 a year, including changeover fitting.
There are, as Im sure you're finding out, literally thousands of online tire dealers, some will only ship to an approved installer, others will ship to whatever address you supply. I ended up purchasing from https://www.tyres-guru.co.uk (UK Based) however the tires were shipped direct from Germany - but with Brexit now in full force I'd be wary of potential import charges for European sourced tires.
I spent hours hunting down online tires - its a minefield. At one point Kwik-Fit online were my P Zero supplier as they had a deal on a buy 2 get £50 off.