You could try Tesco, Asda, Sainsbury. I giggly recommend the meat, fish, and fruit and veg section. Plenty of sports supplements there for bulking and performance. My favourites are steak and pork protein, they're not shakes mind but they do an awesome job. If you need so etching to put in a shaker, go home and use something called a tap. It has this wicked hydration supplement called H2O, all the top pros drink it.
If yiu need any special supplementation, try organ meat for Vitamin A, bananas for Potasium, grapes for extra vitamin C, steak for Creatine and so on
As for peptides and melatonin, no idea where you can legally buy peptides from, but for melatonin just use google. Make sure you have enough time to get a good 9-10 hours sleep in though or you'll feel a little groggy when you get up, that's how I feel when I take it at least. You might feel different though.