Your meter may well have some information on it. Just as important as the maximum throughput on your meter is the pressure required.
This is off the top of my head, so it may be a little out, but mains gas is usually delivered at between 75 and 30 mbar, reduced to 21 by a governer before your meter. Most appliances run at between 5 and 20 mbar (internal governers) so a large appliance might have requirements beyond the norm - you might also suffer if you were to run one of these appliances at the same time as say, your boiler.
You may find it useful to consult with both your local transporter - probably National Grid or one of the newer big transporters, like Northern Gas Networks or whatever (should be identified on your bills) as well as an independent gas engineer (CORGI-registered).
Not a direct answer to your question, but I hope it helps a bit.