... Does it really require lots pof checking and change between UK and North America?
No, if you see the last releases, they are the same worldwide - network is network all over the world and until there are no language packs build in to localize the firmwares, there are no differences in the basics.
But the different D-Link offices need to test these firmwares and what is more important in the new version, the applications. So is the new shareport package that comes with the b09 the version 1.15 which is not the same as the newer firmwares in the north american D-Link routers need to use. So you can't use both devices at a time.
They need to build a newer package based on the shareport version used with NA router firmwares. Or leave it

Also they need to check compatibility with devices only sold in NA - there are a lot of other reasons I can think of.
Last but not least each firmware will run everywhere but if you want support, stay with the version released for your country.