Well that reflects the fact that some US brewers would put Cascade on their breakfast cereal if they could. It's a British style so British hops should be regarded as the natural option. Typically the cheaper hops were used, so you're looking at 2nd- and 3rd-gen Wye varieties rather than premium varieties like Goldings; Guinness use a lot of Target I believe but they're not too fussy.
There's not a lot in it,in many ways Pilgrim is an improved Target with better disease resistance. It's got some character though which makes it more useful as a late addition than Target, and eg Pilgrim extract is used for bittering Cloudwater DIPA. I'm not a great fan of Target, so I'd use it up on this where it isn't going to matter too much.
I love things like Bramling Cross and that's why I wouldn't waste BX on this beer! A BX/Goldings blend makes a great best bitter though.
US recipes (particularly older ones from a time when British hops were less available in the US) throw Willamette at anything that needs a British hop, as an alternative that is easy for US brewers to buy. But it's not always an accurate replacement, it's more about availability than anything else. Put it this way, a British brewer wouldn't view it as a sub for Target.
It's a Fuggle with an extra set of chromosomes that helps the yield, but the fact that it's being grown in different terroir makes a big difference. You can't really replicate the UK climate in the US, spiritually Willamette is closer to Savijnski Golding as a Fuggle family grown in a continental climate.
