A London film makes the impossible his business. When a restaurant owner came shopping for a genuine stagecoach to put outside his premises, the president of Impossible, Ltd., dug one up in an antique dealer’s shop for $700. A newspaper columnist challenged the president of Impossible, David Whitehead, to come up with a half ton raisin-studded custard pudding. This may have been a first in the culinary trade, but two days after the challenge was issued, Whitehead had the pudding at