http://www.vents.co.uk/background.asp

A 9 x 3 brick shaped set would be just under, a 125mm core vent would
be just over. The baffled air core (AC7HM) is 50.44cm2, which is
probably as close as you'll get!

They sell direct if you phone them, and are very helpful and
efficient.

A very good site. I have bookmarked that one.

However, gas appliance ventilation must be permanent, that is not hit or
miss or capable of being shut off or reduced.

It must be fully open at all times (and insect screens are not permitted).

Turn to the plumbing page for suitable vents.