In the uk where do you obtain a heirship certificate?

MY father recently passed away in the uk, he has a plot of land in pakistan, I contacted the housing cooperative where this land is registered, they have asked me to provide them a heirship certiificate, the reason they gave is because i have 3 siblings. My father did leave a will but there was no mention of the land in pakistan. Where do i obtain this heirship certificate in the uk??