It needn't be _too_ expensive.
Here's a reasonable route that, so far, I've found pretty good.
Buy a MacBook - you can get something bigger and more powerful but this will work fine. 800 pounds from www.argos.co.uk - this is your biggest expense.
Buy a Mini DVI to DVI adapter - 15 quid from the apple store on New Oxford Street. YMMV
Buy a 19" LCD screen - this will be essential for debugging. There simply isn't enough screen space EVER for debugging! You should be able to find one for a shade less than a 100 pounds.
Download the latest SDK from Apple.
Register as an Apple Developer
Pay up! Become a paid up Apple developer - 60 quid
2nd hand iPod - if you're not using the mobile network (or audio input) this will work fine - second hand units are around 80 to 100 pounds.
1075 Pounds all in to start developing - Nearly all that cost is hardware.
Get stuck in - it might seem like a lot of money but this is fairly cheap in the grand scheme of things.