What other options are available under the immigration rules and directives, for a British citizen to bring his wife and two young children to UK from Afghanistan? He cannot satisfy the income requirement of £18.600 and I wonder if he can rely on Surinder Singh route or Article 8: Right to Private and Family life based on exceptional cirumstances, compassionate grounds and for the best interest of the children? Thank you.
He can rely on Surinder Singh, but that means he will have to firmly establish residence in another EU country (meeting their standards for residence), bring his family to that country, where they also will need to establish residence, and then reenter the UK with them.
Right to Private life normally has to do with people who are already present in the UK. Since the British citizen could presumbly go to Afghanistan to be with his family, a possibility the UK government will not deny or impede, it really can't be invoked in this case.
The only other possibility I can think of is for the wife to qualify on her own right and be able bring her children with her. There are a variety of possibilities here including student visas and work visas. This is something they would have to explore.