I would suggest you decide what kitchen style you want first and then getting Howdens, Magnet or any other company to visit and produce a design to suit the space available they do not charge for this service. They will give you a supply only price and a fitted price. When you are armed with this you can find an installer, a good start is the MyBuilder website, check the reviews. Make sure you include everything that you want doing ie tiling, plumbing, gas etc, a good installer should be able to advise you of any pitfalls they may encounter when they visit to give you a quote.
A good installer will make even the most reasonably priced kitchen look good, a bad installer can make an expensive kitchen look bad so follow up the reviews.

Getting a kitchen from magnet and using there installation service would be extremely costly where as using a fitter and howdens would be more cost effective with similar results my advice would be to use a small reputable kitchen firm as you would be more likely to get more for your money as most make the kitchens themselves and fit them to

We use Howdens and Benchmark as our kitchen suppliers, we have found both companies very helpful in designing the perfect kitchen for our client's. In the years we have been trading we always advise current and potential client's to obtain at least 3 quotes (yours could be one from an independent kitchen fitter and the kitchen supplier direct). By obtaining more than one price you then get to make you decision based on knowledge, price, experience and know you will get the job done the way you want it.

Personally I would always use Howdens as their kitchens, I have found, are durable and lasting. But at the end of the day it all boils down to personal preference, but I definitely would recommend using Howdens.

Magnets subcontract kitchen fitting out and the install sometimes can be almost as dear as the kitchen, Howdens kitchens are good quality and they usually have a list of local fitters

Magnet are better in my opinion. In all the kitchens I have had through Howdens there would be parts missing, parts broken etc, whilst this can happen it does not make for a smooth job of fitting the kitchen if you are having to go back to get missing or broken parts replaced. Also if they have recommended the installer them it would be for an to them to sort any issues if there are any, if you get the installer it's between you and the installer. Hope this helps.