1. It seems not cut and dried that Sikh's may not eat Halal. Is it not the case that as there has been animosity between the two faiths, that many Sikhs choose not to, rather than being spiritually forbidden?
www.islamawareness.net/Sikhism/halal_for_sikhs.pdf
2. KFC's website says:
"We have chosen a number of stores that are located within areas where we anticipate demand for a Halal offer from KFC. However, we are committed to offering our customers choice and there is a non-trial restaurant in close proximity. The restaurant manager or crew members will be able to provide you with the address details should you need them"
and then:
"We will always only purchase high quality, Grade A, farm-assured chicken from the same leading suppliers used by Britain's major retailers. We remain committed to the same animal welfare standards and continue to insist that our poultry is stunned before slaughter, using a technique called 'stun-to-stun' - a pain free process that makes the animal insensible to pain and suffering. All our suppliers are contractually required to meet or exceed all relevant UK and EU legislation."
I'm sure you are aware that some interpretations - especially from UK Muslims DO allow stunning before killing, making it a lot less torturous.
Frankly I'm surprised you didn't have a look at what KFC had to say
It's harder to know Nando's stance, but this link suggests that they too stun their chicken before slaughter:
http://www.ramadantimetable.co.uk/home/articles/57-is-nandos-haram