Hiya Matt,
Being Stateside myself I would have no idea where to find such bulbs in Jolly Old England.
However, perhaps a little information from this side of the pond will help a bit. The bulbs used in a USA spec EarlyBay taillight are double contact double filament 12 volt bulbs. The attachment lugs on the side of the bulb base are staggered with one being about 2 to 3 mm higher than the other.
A bulb suitable for this application is a 2057.