We also researched kitchen suppliers quite extensively this summer before going ahead with our kitchen project. We decided on a cream gloss range and were considering between Benchmarx and Howdens Kitchens. In the end, we went with Benchmarx Holborn range, as the difference between them and Howdens was that they had soft-closing drawers and soft hinges on cabinets as standard (rather than paid as extra upgrade with Howdens), same 18mm cabinets but with thicker back panels (15mm as compared to Howdens's 6mm on base units and 3mm on wall cabinets) and 66mm void in the back of the unit (compared to 75mm). Howdens were also very slow in providing us with a quote at the time, and I felt I was chasing them instead of the other way round - so, maybe they didn't really need our custom after all.
) have a showroom in Vauxhall (we dealt with Billy Garner, the showroom manager, there) and one in Fulham (although I saw a flyer that they are due to open one in Balham as well, in the Travis & Perkins shop on Balham High Road).
One piece of advice - once you get the quote from a showroom, feel free to ask other showrooms from the same company for their quote on the same design, they are all in competition to win business, so they will be willing to offer discounts or other freebies thrown in. The Vauxhall people were very nice and very accomodating - pre and post service.
Magnet kitchens didn't come highly recommended on other forums, due to poor value for money - quite expensive for the low build quality. If you can afford John Lewis Kitchens, go with them, they are very good, but bear in mind that once you place an order with them, it will take 4-6 weeks delivery. Same for any replacement or other new units that you might need to order on the spot.
We were on a tight timetable, so for us the priority was to get eveything delivered really quickly. With Benchmarx, the advantage was that they would have most of the stuff in their warehouse, so when we needed a replacement due to slightly changing the design at the last moment, they delivered them within a week.