I've since discovered that Megabus.com/UK has not become my only source of credit card consternation. I'm trying to book some form of travel (bus, train, anything!) between London and Oxford and so far, both Megabus.com/UK and First Great Western Railway have refused all of my credit cards.

I've encountered better luck when I booked my London to York trip to visit the National Railway Museum. National Express East Coast Railway declined my Visa credit card, but accepted my MasterCard credit card.

I've left a message with Megabus.com/US customer service to see if they could refer my problem to someone at Megabus.com/UK for a possible solution. I'll keep my fingers crossed!