Beginning to realise this is the answer most of the time lol
Rock auto do offer a fantastic service and as long as you're getting enough stuff, the shipping doesn't throw the price out too badly.
I imagine some of the smaller UK sellers just buy from rock auto and add a markup.
Pro tips: bit of trial and error required but try and get all your parts from the same warehouse (it shows you in the basket if you're using multiple warehouses, which adds more shipping cost).
There's always a 5% discount or so to be had. Google one before checkout
Avoid the normal post options and go with FedEx every time - saves a ton of hassle and I've never had anything turn up late or go missing through this option