The laws in england are a bit different in the USA, and prior aproval of the songwriter is not required to make a Karaoke version legally there.
For that reason many karaoke songs that are on the rare side can be obtained from English companies.
On the other hand to make a legal karaoke version in the usa cant be done for less than $2000 or so per song to the publisher, meaning that unless one has a production line to sell thousands of copies it is not practical.
Many of the smaller more obscure English Karaoke companies sell thier songs throught the Select A track download site.
http://www.selectatrack.com
Of course the Brits tend to have different music tastes so a lot of obscure songs are not obtainable there.