The carcase construction is important and of course varies between manufacturers to manufacturer. Most kitchens are normally 18mm now as
standard but some of the cheaper ranges like B&Q value range are 15mm. The back boards vary massively from 2mm hardboard (ala IKEA) up to 18mm. Howdens I believe are 8mm maybe 10mm, Wickes (Gower) are 15mm. The thickness of the back boards plays a large part in the carcases strength, in general the thicker it is, the more stable and square it is.
Next comes the construction, either 'glued and dowelled' or 'cam and dowelled'. Glued is considered better but and here's the big BUT it needs to have been glued together correctly if it's been glued together by monkeys all out of square you're pretty stuffed. Howdens are glued but I've lost count of the units I use to throw back at them for this reason.
Are the units flat pack or pre-assembled? It can't be underestimated how much time pre-assembled saves you, this should be taken in to account when comparing prices. Wickes kitchens come flat packed but if you buy at TP's trade outlet 'Benchmarx' they are pre-assembled. Same kitchens, pre-assembled, cheaper and in stock no 6-8 week lead time as with Wickes.
Next the hinges and drawer runners, Blum are considered the industry standard now for hinges and drawers, there are other brands which are either as good or better but Blum are a safe bet. IMO anything not soft-close these days is considered a 'budget' kitchen and a downgrade rather than soft-close being an upgrade. Been a while since I installed a howdens kitchen but the hinges were poor with clumsy adjustments, the type of hinges they use were standard 20 years ago, things have moved on and the difference in price between a crap hinge and say a blum soft-close is about £1.80 per pair. It's a no brainer. There is a bigger gap in price between drawers but still soft-close drawers are pretty much the standard.
Service gaps. IKEA kitchens are cheap considering they are Blum hinges and drawers as standard and thick 20mm lacquered doors but then they have poor carcases. Not only a flimsy hardboard back but no service gap behind the units to run services. This means considerable cost altering the services to suit. In addition no room for scribing, so if your walls are out, then so you kitchen will be too.
Doors - Just a simply high gloss door can vary, from 15mm to 22mm (18mm standard) so can the finish from laminated, foil wrapped or lacquered. Other types of doors could be solid timber, MFC, PVC, Veneered. These all affect the price and quality of the doors. So whilst two kitchens can appear similar if you look closely they can be worlds apart.
Carcase colour matching. Many of the sheds will do these 50-60% off units but most will come with either a white or beech carcase meaning decor panels need to be bought. Sometimes you'd buy them anyway if they are decorative but this is where they sting you buy not discounting these items and charging way over the odds, even stuff like kicker boards can be massively marked up. I've seen Wickes sell plinth for £130 a length when in fact you can pick it up for almost a 10th of that price. Other kitchen manufacturers will colour code their carcase at either little of no extra cost. Or you buy doors from one supplier, Like PWS Second Nature, and your carcases from another.
So taking all of the above into account (quality, ease of installation, availability, range and finish) I believe Wickes kitchens (although purchased from Benchmarx) come out on top from all the shed outlets (inc Magnets and Howdens). Don't get me wrong I install many different types of kitchens but if someone wants a cheap to mid-range kitchen it’s a Wickes (Gower) kitchen I would recommend. As I can supply what I feel is a better quality kitchen either cheaper or the same price as all the other offerings in that price bracket.