OK, I've given this some consideration, and here's what I think:
Asking Scotland supporters to support England in the World Cup is like asking Everton supporters to support Liverpool because they're in the FA Cup Final.
I'll certainly be watching the World Cup and hope England do well, but I won't be actively supporting them. The national team I support is Scotland. Just because they're not at the World Cup doesn't mean I should switch my alliegance to a team who are.
Does that make sense?
(.... and not a swear word in sight!)