I don't have the BS546 specs. to hand, but your dimensions match up with the BS546 plugs I've just measured, as near as I can get it!
The reason for not jamming a "Euro" plug into a BS1363 socket is that many such outlets will be on a 30A ring circuit. As the cord attached to the Euro plug may be as small as 0.75 sq. mm, the 30A fuse/MCB in a British panel would not offer proper protection against short circuits.
Another reason is that the contacts weren't designed to accept round-pin plugs, and therefore might not make good contact.
And of course, if you try to jam in a Schuko of French-style grounding plug, there will be no ground/earth connection.
[This message has been edited by pauluk (edited 01-03-2003).]