I'm pretty sure girls in the UK can do similar things girls can do in America..but here's a list :
- Ask your parents for a dollar for every chore you do without being asked (most parents do this from what I know)
- Hold a yard sale with the help of an adult we don't have yards let alone yard sales..
- Offer do help a neighbor like mowing their lawns. Seems like a vintage way but it's still a good way of making money  very unlikely anyone would let a random kid in their garden.
- Dog walk
- Babysitting ... They're 11
-  Help out at a daycare center....They're 11..
-  Hold a lemonade stand  another thing we don't do here
-  Work at the library  again, they're 11....
There's honestly not much an 11 year old can do to make a lot of money :/ but the key is just patience. If you rush things, you'll end up getting frustrated and give up. Just give it some time, kids at that age don't have many options..but I suggest the easiest way is dog walking and baby sitting.