Ticktockmy is correct they can be very strict about taking bikes into London - especially full sized bikes. It does sound like they were being a bit over the top on not letting you in though. Check with your local rail provider on the web before you make your trip.
Central London terminals of Kings Cross and Liverpool Street, on the National Express Line from Ely/Cambridge way, don't allow any full sized bikes into central London, and have rush hour restrictions on some outer towns and station hubs. I think the reasoning is two fold - safety and of course economic; Safety/fire issues if you get caught with your bike in the doorway or on an escalator. Economic because a full sized bike can easily block a train door. Depending on which station the train is at, the trains here use both sides in any one trip. A full sized bike also takes up lots of room meaning they can get more passengers squeezed on - breath in.
On the flip side, they have
no restrictions on folding bikes in or out of rush hour, be they the full sized folding
Montague type bikes, or the smaller traditional
Brompton or
Dahon style. If it folds, it is allowed.
On a positive note - I did see a driver offer to put someone's bike in the cab for someone the other day, so they're not all bad.