A couple of things to check...
1) go to <settings><wireless & network networks><Access Point Names> check if Airtel's APN for internet and MMS is present.
e.g. if you are on the Orange you will see -->
Orange UK [APN=orangeinternet]
Orange MMS [APN=orangemms]
If not to add APN --> go to <settings><wireless & network networks><Access Point Names> click the menu button [i.e. the left hardware button on your Xperia X8] and add your O2 network APNs - if you don't know what O2's network APN is then give the network a call and ask.
Did you remember to enable your data networks?
2) go to <settings><wireless & network networks>and select <data enable>
cheers