The standard of Guinness in the UK has gotten better since the Guinness brewery in Park Royal was closed down a few years ago. It's well know that the further Guinness travels the worse it gets, but the most important think is to have the barrel near to the
pump and the lines must be cleaned on a regular basis.
I never heard of a Black and Tan (the drink) until I started travelling and I don't know anyone back home who drinks them. Blackcurrent cordial (small bit) is often added to a Guinness to get the 'ladies' drinking the black stuff.