Enough sockets. Enough worktop space which won't be taken up with appliances like microwave, toaster, kettle, coffee machine. I hate those new builds where the kitchen worktops are L-shaped, they always feel unfinished, there is nowhere to actually cook. There is nothing worse than feeling hemmed in when you're chopping. Also, "practice" cooking in your layout imaginings. eg. if you have an island near an oven, "practice" taking things out of the oven - does your bum bash the island while you're doing it.
Where will you put your bins?
Get your lights sorted first as the power supplies need to be put in the right places, and the switches too.
Don't scrimp on appliances, spend the most on them, you can always upgrade worktops later.
If you go with Howdens don't use their taps and sinks etc. Pay more for decent quality taps. I didn't used to think it matters, but once you have a shit tap which wobbles around and feels cheap, it GRATES.
Get a full size dishwasher, don't scrimp on a half size one.
If you have bifolds, consider what you're going to do if it's raining all day but you want to open a window for ventilation.
Consider wall space for radiators.