I find this really funny, but we're in Germany and the government publishes a guide to the recommended amount! Here's the current one (Obv it's in euros but I think although the exchange rate is different it would work to just transfer it 1:1)
Recommended pocket money amount for children 2015
Older Pocket money amount
4 to 5 years 0.50 to 1.50 € / week
6 years 1.00 to 1.50 € / week
7 years 1.50 to 2.00 € / week
8 years 2.00 to 2.50 € / week
9 years 2.50 to 3.00 € / week
ten years 15.00 to 17.50 € / month
11 years 17.50 to 20.00 € / month
12 years 20.00 to 22.50 € / month
13 years 22.50 to 25.00 € / month
14 years 25.00 to 30.00 € / month
15 years 30.00 to 37.50 € / month
16 years 37.50 to 45.00 € / month
Age 17 * 45.00 to 60.00 € / month
18 years * 60.00 to 75.00 € / month
But I agree £30 a month isn't a lot for kids' clothing, especially shoes as well. What my mum always did was have a separate "clothing allowance" - I had the child benefit allocated as a second child for this, and she kept it until I wanted to go shopping, but the other way to do it would be to think how much clothing you think she needs in a year - that's summer tops, bottoms, underwear, jackets and how many shoes, winter the same, add a couple of things like swimming costume, decent coat, then go to somewhere mid-priced like H&M (bearing in mind that cheap shoes from places like H&M only last 3-6 months) and add up the cost of all of that, put it all together, and then divide the total cost by 12 - there's your clothing budget for a month. I think it will be more than you think, and she'll be left with hardly anything to spend on herself out of her £30.
I'd do the clothing calculation, and then a calculation of what you want her to be able to buy - bus fare x ?, mcdonald's meal, some accessories, stationery, magazine, cinema, whatever it is she's interested in.