Hi Mark
I'm not clear about the legal aspects, but I can answer some of your questions.
There is a 'British List' maintained by the British Ornithologists' Union which includes all species proved to have turned up in this country without human intervention. This list also includes introduced species which are breeding successfully in enough numbers to be have become permanent additions to our fauna.
You can see the list here:
http://www3.interscience.wiley.com/cgi-bin/fulltext/118619822/PDFSTART
In my opinion the British List is a bit odd, because it doesn't distinguish between birds which exist here in the millions and species which turn up here in ones or twos once every century or so. Some of these birds are surely more 'British' than others, though it would be impossible to draw a line between them.
It's unclear when eagle owls were last native to this country, but it may well have been many thousands of years ago.
cheers
James