Lakeland is the ultimate for kitchen supplies. Their online shop is good, but I love nothing more than flicking through the mail order catalogue and trying to figure out how much more of it I can actually fit in my kitchen. Really good for basics like cake tins that will last.
Bake It Pretty
I love looking on this site, even if I can't actually order anything because the shipping from the US costs an absolute fortune! They stock really cute baking supplies like cupcake cases and piping tips, and their blog is worth a read too.
I only realised about a year ago that Hobbycraft have a baking section, and they're the only shop anywhere near me that stocks coloured fondant and Wilton decorating supplies. They also have a good seasonal selection around Christmas and Easter.
Cakes, Cookies & Crafts
This is an online only store with a MASSIVE selection. Seriously, they have anything you could possibly need for baking/cake decorating.
Believe it or not, Amazon has a surprisingly good selection of baking products. It's a bit hit or miss and I'm sometimes wary of the third party sellers that charge more for delivery, but if you look around a bit there's often a bargain to be had.
Their website is a bit old school, but if you live in or around Leeds, West Yorkshire you should pay this place a visit. I think they're technically more of a trade supplier but the shop is open to the public and their prices on everything from fondant to cake boards are amazing.