I would definitely say DIY Kitchens - really low prices, excellent quality, and you can take your pick of the colours with a bespoke option.
The planner on DIY Kitchens isn't as bad as it seems. I was totally overwhelmed by it and procrastinated for ages. I was so daunted by the prospect of fucking things up.
But I knew I wanted to buy from there so I just gritted my teeth and had a proper stab at it one day. And actually, once you've input your room dimensions and saved it as a template, it's not that difficult at all.
Don't worry about fillers and panels, the planner will make suggestions automatically. When you send your plan to be checked, the lovely people at DIYK will tweak the panels, fillers etc and check there are no obvious problems. You really and truly don't need any expert knowledge at all.
The customer service is excellent - it has to be because of their business model. They know some people find it a bit overwhelming so they'll bend over backwards to help you put the plan together.
Also, another vote for the FB group - it's got DIYK staff in it as well as loads of really knowledgeable customers. The group also has several designers who, for a relatively modest fee, will design, plan, and check your kitchen for you.
We bought two kitchens from them at the start of the year - us and DM. Two totally different kitchens, absolutely lovely. Builders commented on the high quality, which was nice to hear.