In the San Francisco Bay Area, which is kinda sorta the gaming/engineer capital of the world, an entry-level junior game engineer could expect around $60,000 / yr, plus benefits and stock options. Ignoring those benefits, that boils down to (assuming a 40-hour work week and 4 weeks off a year) about $31.25 / hour. 50 pounds a month is, when generous, $100 a month. $100 / 160 hours (4 40 hour weeks) is 63 cents / hour. Minimum wage in San Francisco is $9.50 / hour.
I was working when I was 16 at $7 / hr, washing dishes for a restaurant. I knew how to program a graphing calculator in 5th grade or something like that, and at that age I might have been mildly interested in $100 / month (my allowance was $50 / month).
So. You're looking for kids who barely know how to program, or what?
Now some actual advice... don't bother offering such a tiny amount of money, and learn to program yourself. As a game engineer myself, this sentence of yours: "I have ideas, I can invest time on it, but I need someone else to do the development for me" both made me laugh out loud, tell the game engineer, sitting next to me (as a hilarious joke), and completely and totally turned me off to any thought of ever working on your project ever. Something that's frustrating for me as a professional game engineer is being the monkey who implements other peoples' ideas - ideas you often don't agree with or understand and sometimes from people you don't think are qualified to make game design decisions. But - I do it because I get paid and I learn important things from everyone else.
So, if you take the money out of the equation too... What incentive do people have?
Instead, learn how to code yourself, and then make it yourself.