Don’t order online often - much prefer to go to a store where possible as I quite impatient so hate waiting for a delivery to come.
Although these are the places I typically order from online:
Pets at home
Zooplus
Rodipet (German company but ships to the Uk - for €4.95/~£4.26)
Minipetz -
https://www.minipetz.co.uk
I have in the past ordered from petplanet and viovet - shipping can be a bit pricey for my liking.