i think orange has a 750mb data allowance, i dont think 02 does.
however im constantly checking my emails, facebook all day, and use msn on my iphone, and occasionally update apps via 3g, used maps in london alot, and im lucky if i use more than 100mb of data a month.
what you got to look at is the majority of apps are designed to use a minimal amount of over the air 3g data or over the air data.
even if you use radio streaming apps (which i do) you would struggle to use 750mb of data a month.
also im on o2, but every major city or town will have access to a free wifi hotspot (o2 provide theres via the cloud & BT openzone, not sure about orange)
id say you needn't worry about getting this data free, its gonna b hard to use that ammount by the points raised above.