Coombsy
What a can of worms question!!
Everyone has thier own principles when it comes to listing and discussions about what you can and can't tick can often get quite heated, but basically there are no rules - it's up to you.
There are however a few basic principles that i think most serious listers would agree on.
The official British List is "owned" & maintained by the British Ornithologists Union (BOU) and can be viewed / downloaded at their website:
www.bou.org.uk
This is split into 5 categories: A, B, C, D & E (this is where it starts to get a bit complicated!). Cat A are genuinely wild birds recorded in Britain (as a geographical unit - Englan, Wales & Scotland - NOT N. Ireland or Eire, or Isle of Man or Channel Islands - still with me?) since 1948 (i think); Cat B is for birds recorded in Britian in a genuinely wild state, but not since 1948; Cat C is for birds that now occur in Britain in a wild state but are the result of introductions or are feral populations.
Categories A,B & C make up the "official" British List. The birds in these categories are the only ones that most birders will tick on their British Lists (currently standing at 573 species with todays addition of Chestnut-eared Bunting).
Category D is a holding category for birds that would otherwise be included in Cat A, but there is reasonable doubt that they have occured in a wild state.
Category E is for Escapes, introductions & birds that have bred in Britain, but thier populations are not self sustaining.
See the BOU website for a full description of these categories.
Right, thats the basics - still awake?!?
Some birders use other lists (eg UK400) that have several species that on the official list are treated as subspieces and/or escapes, but are deemed "tickable" on these lists.
However, come birders use the BOU list, but only tick Cat's A & B and this where it can start to get a bit muddy.
Most of the birds in the Slimbridge Collection would probably be on Category E of the BOU list even though there may be genuinely wild individuals of those species in Britain, and this where it comes down to you.
If you feel comfortable ticking say a Pintail from the Slimbridge collection, even though it is obvioulsy not wild then that is up to you. Over time your list will evolve and many species that you would have once ticked will be removed (eg a few years ago i removed Golden Pheasant from my llist becausse i have not seen it in its feral population (Cat C), but i had seen it roaming around Stapleford Park when i was about 12 (Cat E).
I Hope this: a) helps; b) doesn't confuse too much & c) doesn't put you off!!
I'm sure there will be loads of others on here who will also have their own opinions on lists and i guess that is the main point - it's all down to you as an individual what you want to tick (or not!)
Cheers
Mike
edit: Rob posted whilst i was typing this and has summed it up much more succinctly!!