I've fitted quite a number of kitchens, over the years - mostly supplied by Howdens.
Howdens quality is OK. Not poor, not amazing. One thing that does piss me off about them is that they almost never deliver a full order on the due date, and it's a pain in the arse having to chase them up for missing components when you should be concentrating on fitting the damn thing.
Having said that, they have some really decent, helpful staff working at their branches.
Avoid B&Q off-the-shelf generic cabinets - they are made somewhere in Eastern Europe and they are appalling quality. Absolutely shocking.
Diykitchens.com are, I would say, one notch above Howdens, in terms of quality, but you can't just drive down to a local branch (unlike Howdens) if you have a problems or need to replace a damaged component, etc. However, if you are willing to tolerate the inconvenience of ordering online, then Diykitchens.com are worth considering.
I've never fitted a Wren kitchen, and they pitch themselves as better than average, but I've seen quite a few mutterings of discontent on various kitchen-fitting forums, from past reading.
Whoever you go with, for the love of god, please don't make the rookie error of trying to economise on worktops. Economy laminate is garbage, not fit for purpose, and I absolutely guarantee you will regret it if you buy cheap worktops. Approximately £120 is the beginning of acceptable laminate, with quality improving as you climb higher in price. Below that, you'll be throwing your money away on paper-thin laminate with sub-par blistered post-formed front edges. If you stand about 3 metres away from budget laminate, in the right light, you will see the front edge of the laminate is blistered / not fully glued to the chipboard substrate. This makes it insanely fragile and it fractures when it gets a light knock with something hard. Budget laminate also has unacceptably thin top surface layer that doesn't yield a reliable result when routing joints etc. It's actually so absurdly thin that you can often see the texture of the underlying chipboard showing on the top surface, instead of it being glass-smooth. Imagine if you put a paper-thin slice of cheese on a ryvita cracker - it wouldn't stay smooth.
Another thing to watch out for with chipboard laminate worktops is that the ones with a 100% flat front edge (I mean no post-formed roll-top / radiused front edge) may look nice, but, over the longterm, they have a tendency to allow water to penetrate and then the front edging tends to 'blow'.