I am going on holiday to spain next month and am taking my N800 with me. I have read that yoigo offer the cheapest GPRS/3G internet in spain. Can anyone ship me a prepaid sim to the UK? And how much would it cost?

According to this page:

http://www.yoigo.com/preguntas/tarjeta_preactivada.php

you can buy a preactivated sim at a newsstand or at a "telecor" shop, but you have to provide your data, and I doubt they'll sell one to you if you're not living in Spain but you can always try.
The cost is 10€, which is also the prepaid amount held by the card.

Edit: you can find the location of the nearest telecor shop here.

Disclaimer: I'm not affiliated to neither yoigo (in fact I don't like them) nor telecor (I don't even know who they are), I'm just trying to help.

Thanks, yeah thats what I thought. Is there anyone in Spain who could buy me one and post it to me in the UK? I could pay them by paypal.

Don't need one now, bought one off Ebay.