We got a Schuller kitchen, through KDC UK. They're not based nearby, so might be a bit of a trek to get to their showroom, but the service was good, the installation was great, and the price was good value.
Definitely will be more expensive than Howdens and Magnet, so if those are your budget then don't consider Schuller.
We chose Schuller after considering several different German kitchen brands. We chose Schuller for the best price to quality ratio.
Schuller is also sold by several other kitchen designers in London.