Get all your bits and pieces together so you're ready to start at short notice, ie next day - as mentioned a CSCS or preferably ECS card, basic hand tools, PPE (hard hat, hi-viz, boots, glasses, gloves, ear protection). Most sites will provide gloves, glasses and ear protection, some will give you a hard hat and hi-viz, some won't, or at the very least will expect you to have your own when you start.
I would add that it looks better if you go dressed for work instead of just wearing old clothes. You don't have to spend a fortune - last time I checked you could buy a plain polo shirt from Primarni for ÂŁ2.50.
Some will argue about the last bit and say the way you dress has nothing to do with how good your work is or that it's nobody else's business if you've got a dog in the van, and fair enough if you can get away with that, but if you're starting out on the bottom rung of the ladder it's best not to rock the boat.