As this thread shows, GSF follow the same stunts as ECP, and I too have found them (like ECP) to be quite unhelpful, both with pricing (they won't) or finding the correct part.
I was, of course, talking about the independent factors. Obviously, we have none around these parts, but I work in MK, and frequently all over England, and always find calling them up helps if going for a cheeky discount. That also allows that unanswered question on all online shops, "do you actually have it in stock at your premises".
I found CAF, local to you, used to be good like that, but have gone down the pan since taken over a year or so ago.
But I always use the phone, and go for the cheeky discount before driving anywhere. Even from the dealer... ...which some reasonable results at Jag.
Have a well known VAG parts distributor next door at work, so occasionally pick up VAG stuff from them. Always ask, and occasionally get, a small discount.
As for tyres, I normally try to get most places to at least match Black Circles. The local Black Circles fitter here is, as I'm sure you know, Tyre Warehouse. And I wouldn't ever let them touch any car of mine ago. So I normally ring up chunky in town, tell him the Black Circles price, and he will generally call me back a few mins later saying he can match. As do, IME, WIM.
But like threatening to cancel Sky to get a good offer, I'm perfectly prepared to not get what I want and go elsewhere. Same with breakdown and so on. Can't do that online.
YMMV, obviously, but I always find its worth the ask sometimes
HTH