As far as I can tell, you can find the Tesco mobile balance by dialling *#10#. If so, you can do the same thing through AT commands like this:-
AT+CUSD=1,"*#1345#"
OK
+CUSD: 2,"Your ASDA Mobile credit is now #0.25. If you have purchased a bundle, please call 2732 for further information.",250
This example is from a few years ago, and I think that ASDA are now using a different network so that codes don't work, but you should be OK with Tesco as long as the *#10# code works.
The first line is the command that you need to send. You will need to change the code to *#10#.
The mobile device will reply with "OK" almost immediately.
The last line appears maybe 5 - 10 seconds later, when the network has responded to the request for credit. You will have to see what form that is to work out how to find the credit from it.