The Beeb's iPlayer now offers image quality to rival freeview, but watching an hour programme now means using 600MB of your download allowance, not the prior 300MB.
It's automatic: Open 'the large video window' and it'll use this quality if possible, though you can switch to 'use lower bandwidth version' for the old quality.
HD Content: High def is now also available if selected, but then an hour uses over twice as much again; 1.5GB and requires a fast PC & connection.
Check if your broadband's limited. Many impose monthly download limits eg 2GB, and after that charge per GB, so regular web viewers should consider an
unlimited download deal - see the
Cheap Broadband guide. Plus, install a bandwidth monitor like
Rokario to check your actual usage.