I was going to suggestyou try the Dive-In shop at the Lakeside Centre, King's Norton, but it looks like they're ceasing operations there, owing to redevelopment of the site, and moving all their operations to their diving centre at Dosthill Quarry. Air fills are available, but last time I was there they could only fill to 232 bar.
http://www.divein.co.uk/birmingham.htm
http://www.divein.co.uk/about%20dosthill.htm
Their contact details are:
Dive In Birmingham:
Dive In
Scuba Diving Centre
Lakeside Centre
Lifford Lane
Kings Norton
Birmingham
West Midlands
B30 3NT
Dive In Dosthill:
Dive In
Dosthill LTD
Church Road
Dosthill
Tamworth
B77 1LL
Tel - 01827 281304
Fax - 01827 281304