I work in the kitchen industry and can give yo some inside knowledge.
The average cost of a quality kitchen in the UK is £14k. This obviously depends upon the size.
Most good fitters are fully booked for at least six weeks.
Wren - as a customer you are paying for a big glossy showroom, lots of sales staff and a big premium for interest free credit. They load the cost of the kitchen and let you pay it back over a period of time. Yes they are run by an ex MFI director. You need to arrange your own fitter, but they can refer one to you from their bank of fitters.
Howdens was the trade arm of MFI. They are not open to the public, but supply the trade. Prices very much depend upon the discount that the fitter negotiates. Again you need a fitter to be able to buy a Howdens kitchen.
Wickes are decent kitchens in a variety of price ranges. They are part of Travis Perkins as are Benchmark kitchens. However again you will be subsidising a large showroom with a big sales team.
Fitters are a cheaper way to get a new kitchen, however a good one will be booked up for months and they wont always be as good at designing a new layout. They will order doors and cabinets from a variety of manufacturers.
If you fancy a new kitchen but are happy to keep the existing layout, there are kitchen refurb companies. These can replace doors and trims and also worktops. This isnt cheap but can be a good way to get a better quality kitchen at a reasonable price.
In all of these kitchens, price is a key. The lowest price kitchens such as Ikea, are very poor quality and I would avoid. the DIY stores can supply low cost kitchens, but again can be poor quality. Independant design studios are expensive, but provide a high quality design and fitting service. The average kitchen should last 7 - 10 years, it really depends how long you expect to stay in the house.