Just to murkify the water even more...

The UK government has decided that ones ethnicity is NOT necessarily derived from your ancestry for purposes of census data.

If, for example, you were born in Pakistan, but emigrated to, let's say Fife, when you were 3yrs old and so were brought up in Scotland, learnt to speak Gaelic at school, and consider yourself Scottish as that's all you've ever known, you could legitimately classify yourself as Scottish. So 'officially', by extrapolation, you could decide to claim whichever of the Clans you felt the strongest tie to.

I know with my own clan, MacLeod, even if you are not a blood relative, but your family sought service and protection with them, you could legitimately claim ties to the clan.