ChrisG wrote:
Whats the reason for using say Goldings or Fuggles with AA of 4% as first boil hops? Would it not be more cost efficient to use say 20 grams of Target instead of 50 grams of Goldings to bring it to the same IBU?

Yes, it would be cheaper and more efficient to proceed the way you suggest and this is how many big breweries brew their ales. However, it has long been said that the bitterness associated with high alpha hops can be somewhat harsh and on this basis the best ales utilise the best hops.
I suspect that much depends upon the type of beer you're aiming to produce. The likes of stouts probably don't really care!
I tend to use both Fuggles and Goldings in my darker bitters and have concluded that flavour aside, they do have different bitterness profiles, which enables me to tweak my brews. Goldings impact more upon the rear of the tongue and the bitterness is ever so slightly metalic, but mellows beautifully with conditioning. Fuggles meanwhile seem to impact more on the front of the tongue and the bitterness isn't so spikey. By blending the two I can create a real mouthful of flavour.