all the way along the worktop, all round the room, have a switched double socket (or a switch to control an appliance below or above) every 600mm in a horizontal row. This is actually easy to install and economical on time, chasing and cables. If your cabinets are 500mm wide, spacing at 500mm might look neater.
Remember that the cooker hood and/or extractor, and working lights, might be above the worktop. Run a cable upwards vertically to a flex outlet or socket, with a (probably 5A) FCU in the row by the worktop. For under-counter appliances, have an unswitched socket vertically below, and a switch in the row. If for some reason you are not having a socket, you will need an FCU with a fuse appropriate to the appliance. Fuses in appliance fused UK plugs blow, on average, approximately never. So it is not tiresome to have them under the counter. Cables can be run vertically or horizontally. Never diagonally.
About 200mm above worktop height, to clear your tiles or upstand, and be out of the way of splashes.
On the island, put them under the worktop so they are visible and easily accessible, but not prone to spills.