Howdens only sell to the trade, and they have a variety of kitchens of different qualities. They are generally quite sound and in standard UK sizes. They are reputed to be a bit better than Magnet or Wickes. I got my last kitchen from Magnet because I particularly wanted solid hardwood doors, and didn't like the Howdens range. The prices are much of a muchness, there's always a half-price sale in the retail outlets.
Ikea are not standard UK sizes, and installers frequently complain that they do not have the usual gap for running pipes and cables behind.
As Ikea are not the standard size, you may find it more difficult to buy additional units in future that look as if they match. I don't know if it is more difficult to buy replacement doors off the shelf when you want a change (there is usually no need to go to expensive bespoke replacement door companies, you can buy standard sizes from manufacturers, or brand-new surplus stock or discontinued lines on fleabay).
The most important thing in having a kitchen fitted is the skill and care of the installer. If you can find a good local joiner, and preferably pay him day rate so he isn't tempted to skimp and bodge, he can make a budget kitchen look like a million. The boxes themselves are very simple. Rigid is much better than flatpack. The worktops, cornices and accessories wil make the difference. If you have standard sized cabinets you can add rotating and slide-out fittings later if you want.
Have metal, not plastic or wood, drawer boxes.
Get plenty of pan drawers in the biggest sizes you can.
Don't buy your appliances from the kitchen company.
If your hot water comes from a tank, buy British taps like Bristan, not stylish Italian ones with poor flow and difficult parts sourcing.
Have more sockets than you think you could possibly want. Have appliance switches above the worktop feeding sockets below.
Have a cooker hood with a proper extractor, not a recirculating hob which serves no purpose except as a noisy ornament.
Have a hole made for a tumbledrier trunk.
Have your plumbing and electrical work done by pros. Kitchen fitters are not plumbers or electricians, and are notorious for poor-quality and non-compliant work.
Do all your plans before you start talking to people, and do all your revisions before you place an order, and definitely, absolutely, before starting work.