The 8k race is probably an attempt by the organizers to create something to appeal to the home crowd: an event where the best British runners will compete as a team and although the competition from the Americans will be challenging there is a chance they can win. If they just had the 4k at the British and American athletes ran that, the British fans would probably get bored watching the Africans dominate, perhaps the odd American being able to keep up, and the Brits being nowhere in sight.
However, they want to retain their status as a world class event, hence they keep the 4k race.
In my experience while Americans and Brits both get beaten by Africans, Americans have an attitude of wanting to fight back. Brits do not. Hence the Brits have set up this meet to favor the attitude of themselves.
Although who knows, maybe on eightlane.com they are having this same discussion right now. Maybe it's the race organizers but not necessarily the fans over in the UK.