I hope no-one minds me contributing to this thread. I am a independent kitchen fitter and supplier. First of all I absolutely not interested in promoting myself, I'm way too busy. I just thought I might add my own experience as I know from my own customers how much pain they go through when choosing a kitchen.
I'll start with a negative. Avoid B&Q and Homebase like the plague. I talk to a lot of kitchen fitters in real life and online and those two are guaranteed to make professional fitters tear their hair out.
On to Howdens. They are OK, they have moved on a bit since the MFI days and they are a reasonable bottom to mid range kitchen. The cabinets aren't amazing but they do the job. Their great strength is the amount of stock they hold and even if they don't have it a nearby branch will. They do however charge accordingly for this as all that commercial space isn't cheap. Most builders and just about any tradesmen seems to have an account with them which is actually a disadvantage. It means that any old Joe can tell you that they fit kitchens as Howdens do all the survey, design and pricing for them. It very often means that people that can't really fit kitchens end up fitting them and sometimes badly. The biggest problem with Howdens is the cost. I have almost stopped supplying them (not done one in last 12 months) as they are too expensive. Their ridiculous discount system is actually very clever and they use it to their advantage. I can get a decent price if I go up the road to Magnet Trade, get a price from them (mentioning that Howdens are pricing it too) and then take that price back to Howdens who evntually give in. I can't be bothered with all that so I go elsewhere now.
Someone mentioned Benchmarx. I have no personal experience but other fitters tell me good things about them and they seem to be reasonably well respected in the trade. I think someone said that Wickes use Benchmarx and I think that's the case. Wickes are probably OK as well but I know that my local store only moves kitchens when they have a sale on. Wickes sales do tend to be real ones unlike B & Pooh.
Ikea certainly polarises opinions and from research I have done in the past seem to be very cheap. Their range of cabinets is very limited compared to most, but for anyone on a budget you can't get near to them for value. I can't imagine they will actually fall apart and a lot of Ikea's bad repuation in the trade isn't to do with quality, it's to do with design. Their cabinets do not have a void at the rear for services (i.e plumbing pipes and electrics.) It's good for the end user as you get more space but not for the fitters. It's fine if you can run all your services below the cabinets which can be done. However most existing UK kitchens will have existing services behind the cabinets and these will all have to be moved if you opt for Ikea. The cabinets also don't allow for adjustments for concave or conxex walls. Normally if this happens you can trim the back of the cabinets to make sure that a 600mm worktop will still have sufficient overhang at the front. You can't do this with Ikea units.
Magnets has been mentioned. I don't use the retail side as I think they are ridiculously expensive and although Magnet Trade is cheaper, my local branch treated me like a numpty once too often and I have voted with my feet.
Anyway hope that helps. Re doors delaminating, it can happen with almost any supplier. I use a supplier that I would consider better than all of the above and you still hear of problems. Any vinyl or foil door can be susceptible to delaminating especially if it close to a source of heat or steam.
Re fitting, unless you have a really good personal recommendation from a friend, you're taking pot luck. Always remember that if you use a finance package to fund everything, there are disadvantages when it comes to snagging lists at the end of the job. They will already have been paid in full and there is no incentive for them to keep coming back to rectify faults.
If you can, ask to see someone's existing work. All of us say all the right things when we're touting for work but talk is cheap. Don't employ someone because they're a nice guy either, it doesn't make them a good kitchen fitter. The grumpy guy might be your real friend. He might only be grumpy because he's tired of women telling him what a nightmare it is having a kitchen fitted. We know and it's actually insulting.
Whoever you go with make them plenty of cups of tea and coffee, it's the best money you will ever spend and too often overlooked. Most of all don't treat your tradesmen like a servant, it won't get you a better job!!
Hope that helps even if it is not at all definitive. It's a nightmare!
Oh and I neally forgot, I use my supplier because they are incredibly reliable and offer a vast range compared to all of the above at a better price and quality. Before anyone asks I'm not going to mention them as it's a trade secret and I'm keeping it that way.