As one of the people banging on about Ikea + fancy doors on another thread, I won't repeat here what I said there (or will try not to at any rate!), but I think Ikea vs Howdens vs DIY etc is largely down to personal preference and what your builder/fitter will work with, if you're using one.
Mine is happy to install Ikea and used to the pipe work issue, supply methods etc. Last time I had an IKEA kitchen fitted the builder and I went to Wembley together to buy it, I paid, he collected.
FWIW, Naked doors have an option for Howdens door dimensions on their website as well as Ikea, but I like the flexibility and price transparency you get with Ikea (as for @SallyLockheart, for us the Ikea dimensions work really well in the space we have available).
I did a very rough price comparison on all the main suppliers of Ikea-sized doors, using a 60 x 80 cm plain slab door as a guide, then costed up with Naked and Custom Fronts (more legwork neither the former as you have to do it yourself, whereas Custom Fronts gave me a quote from my Ikea plan).
Am in the process of getting a price from Foxstow as well, which I expect to be a bit cheaper than Naked as the doors are MDF not wood.
Basic prices: (bearing in mind I did this a few months ago now so prices may have gone up)
Painted slab doors 60 x 80 cm:
Foxstow £84
Naked £93
Noremax £86
Husk £162
Supperfront £91
Custom Fronts, Plykea and All About the Door don't give prices for single doors online.
The choice for me comes down to:
Door style/ material (slab/shaker/other (eg tongue & groove)?
solid wood/ ply/ fenix/ MDF?
Painted or wood finish?
Can they supply (do I need?) custom units eg larder as well as doors?
Colours, own range/ colour match?
Samples available? If so at what cost?
Lead times/ supplying from UK or EU?
I've had a lovely play with the DIY kitchens planner, which is great and shows you your layout in F&B colours if that's your bag (it is mine), but I suspect I will stick with dollied-up Ikea for the internal storage even though it will end up more expensive (and also because what @floppybit said)