The shorter way:

1. Call 44555

2. Choose option 1

3. A text message will be sent to you, stating about how much minutes you have remaining, and how many sms and mms you've sent for that month.

The longer way:

1. Call 191

2. Choose option 2-1-1

3. The same text message as above will be sent to you.

Note that the information will not include any calls you've made or sms/mms you've sent in the last 72 hours.