There are 402 Nandos restaurants in the UK but only 74 serve halal chicken. In edinburgh there's only one which is located in ocean terminal and is very far away from the city centre. As a Muslim it's hard to find places where halal meat is served and ever since the halah branch opened up in ocean terminal it became a very popular place for muslims to eat.  Since Nandos is becoming such a popular eating destination and there are already existing halal Nandos restaurants it would make sense for them all to become halal. Why would you only make some of your restaurants halal and not others? Halal meat is also the most ethical way of sourcing meat. So even if you weren't Muslim, your meat would ethically sourced. 

I think making every Nando's halal would be the most logical and inclusive way forward.