I believe you have a Sphinx mark rather than seated lion. I have seen this exact mark (plus several other sphinx marks) attributed to silver plate manufactured by John Batt & Co of Sheffield. In particular, there is a dish for sale on eBay now that has this mark plus the words "Sheffield Reproduction" .
If you read the following article on the history of the Birk business you will find towards the bottom, that Birks actively purchased items from other countries to resell in their retail outlets. http://www.925-1000.com/forum/viewtopic.php?f=38&t=19623