Good experiences with delivery (ie. would order again)
Dabs.com
Overclock.co.uk
SVP - Blank Disc Shop
Crucial Technology
Reasonable experiences with delivery (would think twice before ordering)
Choice Stationery Supplies
Overclockers
Ebuyer
Dabs - everything arrives within a few days of ordering in good condition. No real gripes about their couriers, parcelforce. Customer service, however, is another story altogether.
Overclock - again the same with Dabs about their couriers, Business Post and packaging standards. They never seem to have replied to any emails I've sent them, but their RMA process for me had a quick turnaround time, but sort the comms problems via email!
SVP - again same as above with couriers. Emails all responded to fairly quickly so a

for them.
Crucial Technology - same with all others for couriers - they use Royal Mail special delivery which isn't too bad around here. Can be a bit slow to respond via email but at least they have a free phone number.
Choice stationery supplies - not too bad up until recently when I have had a few packages go missing or when I've ordered the wrong item. Never had a reply to my missing package emails and when I tried to email them to cancel something to ring some 0845 number - and when I asked the reason as to why they couldn't deal with cancellations via email, they just copied and pasted the text asking me to phone them from the previous reply.
Overclockers - apparently they are legendary for their customer service, or rather, lack of it. Bought a HD from them a while ago and the box that it arrived in was pretty tatty.
Ebuyer - Citylink, their couriers are not much use. Ebuyer's website is nice to browse through and have got good order tracking stuff but they have no phone number and you can't email them directly - you have to use their enotes system, which, thankfully (touch wood) I've never had anything break. Yet.
They also sent via Recorded delivery my graphics card in one of those thinly padded envelopes - generally not a good idea. Granted it only cost about £50 but I believe that if it got nicked in transit or damaged I wouldn't be covered for the max amount and be left out of pocket - IIRC it's a max of £28 compensation for lost or damaged items.
So, a mixed bag really.