Not bought anything new for quite a long time now tbh.
Due to certain horror stories I've heard, I won't use Scan for motherboards, but would for pretty much everything else if they're cheaper.
Happy to use Aria for anything and their prices are often cheaper, but their stock is far more limited than other places which is often an issue if you're trying to make a larger order.
I tend not to use OCUK unless I absolutely have to. Their prices are nearly always more expensive - even their deals of the week are often more expensive than other places, and also they lack postage options which ruins it the most for me. Anything of reasonable size and you're forced to use £10+ couriers (I think this is still true?). That just completely puts me off when I can get free delivery from a fair few other places.
Ebuyer are actually one of my favourites, but again due to the lack of free next day delivery I can get with some places I'd rather save money and go elsewhere. Having said that, their delivery options are amazing. You can order up to 11pm and have it first thing in the morning, and that's still only £10 or so. Also, their RMAs are amazing. They pretty much have a 2/3 day turn around, and do everything themselves so never have to resend things off to the manufacturers. I've sent back motherboards before and even the first STX I had, they've tested them the day they got them back, and either given me store credit or sent a new one out that same day.
Specialtech's another good one. Great range when it comes to cooling, and fairly competitive prices. Occasional issues with them being a small store and their website isn't as as up-to-date/sophisticated as it could be, meaning sometimes it can say something is in stock, when it isn't, then it delays your order because of it, and due to a lack of communication you aren't notified. If I order from them, I normally put a basket together then call them up to make sure everything is in stock before placing the order. Other than that though, they're pretty good.
That's most of them gone through I think, happy to use Amazon, and CCL's another one of my favourites at the moment. Never had an issue with them.