Be Quiet have released a Bronze-rated 300W SFX PSU at £60, which is what you need to fit. Silverstone do a gold-rated modular 450W SFX PSU, but that's about £90. I have one running my SG05 ITX Titan rig and it's not the quietest power supply in the world, but it is rock-solid bulletproof.
Then again, you could look at buying the QS101 from somewhere else that has a PSU included in it, but I couldn't comment as to the quality of it.
To be honest, by the time you've dropped £60+ on a PSU to fit, you'd be better off considering a different case.
What is your exact spec?