YMB, In response to your query - "But do the SNP put up candidates in the UK? try clicking
http://en.wikipedia.org/wiki/List_of_Scottish_National_Party_MPs_%282010%E2%80%93present%29
You may not have noticed the recent independence referendum, but the result of that was that Scotland remained in the UK. As I pointed out in an answer earlier here, even
now they have more party members than any other political group except Labour and Tories.
With six Westminster MPs currently, they are quite likely to be more numerous than LibDems after next May! That is especially so given that they'll probably take a few seats off Labour, too, after the referendum!
Accordingly, they have a greater right to be part of the debates than Greens or UKIP for sure.