Cost effective 3G or Data Card service?

From Abingdon England,

I have a caravan in Blue Anchor Bay, Somerset, England and I need to have E-Mail contact but I am not happy to pay close to £400 to have a telephone land line put in.

I understand that there is a gadget called a 3G Data Card that I could put in a Laptop which will allow me to download and send E-Mails.

However, as hard as I try I can not find a way to work out which is the most cost effective 3G service. Can anyone point me in the right direction to find one?