DD did a PGL type residential and is also a Scout, muddy activities are well know in our house.
For outdoor activities - see if you can get decent hiking trousers second hand. If not, defintiately no jeans, thick leggings are better. Jeans can be used for evening indoor activities.
T-Shirts and jumpers, fleece is good as it's thinner and warm and dries quicker than a thick hoody. 2 T-shirts per day in case they get all muddy.
2 pair of socks and underwear per day.
Two pair of trainers at least, can you get you hand on walking shoes/boots? DD had to bring a pair of indoor shoes, slipper or similar, requirement of the center as they didn't want the mud everywhere.
DD took a small digital camera with her, dangerous as she coulod have lost it but she came home with great photos. Phones were a no-no as were game consoles.
They were encouraged to bring playing cards and at least one book and a torch.
Bin bag for muddy and wet clothes.
Wash kit with sanitary products, you never know if it starts.
If you can, get a travel towel, takes less space than a normal one in case you need to bring your own.