Whats it to go with and do you need to play games on it? Anyway a 5450 or 6450 are around your only options and a quality power supply is over £35 so if you want one to use when you upgrade thats your whole budget gone.
That I think is the best, your budget is too low mate. It won't be far if you have Intel HD 3000 or AMD integrated 6410D. Or probably your old 9400GT. You can however save more money and get cards+PSU when you there is Radeon 7xxx coming up.