I just had a glance and found these differences
Greenland/Eurasian White-fronted Goose vs. Greater White-fronted Goose
Tundra/Taiga Bean Goose vs. Bean Goose
Snow/Ross's Snow Goose vs. Snow Goose
Dark-bellied/Pale-bellied/Black Brent Goose vs. Brent Goose
North American Wood Duck vs. Not on list
Cinnamon Teal vs. Not on list
Marbled Duck vs. Not on list
American/Stejneger's White-winged Scoter vs. White-winged Scoter
White-headed Duck vs. Not on list
Cory's Shearwater vs. Cory's/Scopoli's Shearwater
Bulwer's Petrel vs. Not on list
Madeiran Storm-Petrel vs. Not on list
European White Pelican vs. Not on list
American Least Bittern vs. Not on list
Little Blue Heron vs. Not on list
Greater Flamingo vs. Not on list
American Bald Eagle vs. Not on list
Eurasian Griffon Vulture vs. Not on list
Eurasian Black Vulture vs. Not on list
Red-shouldered Hawk (ship-assisted) vs. Not on list
Booted Eagle vs. Not on list
Western Swamphen vs. Not on list
Azorean/Yellow-legged Gull vs. Yellow-legged Gull
Iceland/Thayer's Gull vs. Iceland Gull
Little/Least Tern vs. Little Tern
Elegant Tern vs. Not on list
North American/Black Tern vs. Black Tern
European Eagle Owl vs. Not on list
White-rumped Swift vs. Not on list
Northern Flicker (Ship-assisted) vs. Not on list
Middle Spotted Woodpecker vs. Not on list
Acadian Flycatcher vs. Not on list
Eastern Kingbird vs. Not on list
North American/Horned Lark vs. Horned Lark
Yellow/Eastern Yellow/Black-headed Wagtail vs. Yellow Wagtail
White/Pied/Amur White Wagtail vs. Pied Wagtail
Siberian Accentor vs. Not on list
Siberian/Caspian Stonechat vs. Siberian Stonechat
Grey-cheeked/Bicknell's Thrush vs. Grey-cheeked Thrush
Western/Eastern Subalpine Warbler vs. Subalpine Warbler
Desert/Lesser Whitethorat vs. Lesser Whitethroat
Two-barred/Greenish vs. Greenish Warbler
Siberian/Chiffchaff vs. Chiffchaff
Mugimaki Flycatcher vs. Not on list
Great Grey/Saxaul Grey Shrike vs. Great Grey/Southern Grey Shrike
Indian House Crow (ship-assisted) vs. Not on list
Daurian Starling vs. Not on list
Mealy/Northwestern Redpoll vs. Common Redpoll
Hornemann's/Scandinavian Arctic Redpoll vs. Arctic Redpoll
Parrot Crossbill vs. Parrot/Scottish Crossbill
Blue-winged Warbler vs. Not on list
Canada Warbler vs. Not on list
Spotted Towhee (ship-assisted) vs. Not on list
Field Sparrow (ship-assisted) vs. Not on list
Fox Sparrow vs. Not on list
Chestnut Bunting vs. Not on list
Red-headed Bunting vs. Not on list
Rusty Blackbird vs. Not on list
Yellow-headed Blackbird vs. Not on list
Neither list Dalmatian Pelican, Red-footed Booby, Daurian Shrike, Lammergeier. Most birds missing from the BOU list are Category D (which I think should be countable), some have yet to be accepted (Thayer's Gull (?), Acadian Flycatcher, Eastern Kingbird, Siberian Accentor), some were ship-assisted (Red-tailed Hawk, Northern Flicker, Field Sparrow, Indian House Crow, Spotted Towhee), some occured only in Ireland (Slaty-backed Gull, Canada Warbler, Blue-winged Warbler) and the rest are splits.
I must admit I prefer the Club400 way, eventhough I don't support a few of the splits they do (Redpolls, Wagtails, Brants) and allowing to include ship-assisted birds.
Now on basis of this comparison I guess it should be possible to convert this years attempt to the BOU list
Maffong