If you mash low, use a large enough starter, and rouse the yeast with a little shaking when it starts to drop its krausen, you can get 75% attenuation with 1968.
For IBUs, I would say Fuller's is probably in the 35 to 40 range. The reason I suggest that you not use Northern Brewer is that, at least to me, it has a kind of minty flavor that seems out of place in a Fuller's clone.
For SRM, I'm guessing somewhere around 12, but that's tricky to estimate just looking at the glass. 16 is deep red-amber, which Fuller's is certainly not.
As for the base malt, Marris Otter is closer to Optic than Golden Promise. The flavor of pale malt comes from the barley variety as well as the conditioning it goes through, so there really isn't any way to make one malt variety taste like another.
That recipe looks better, except that English crystal malts taste very different from American ones, and as far as I know, there is no English maltster that makes a 90L crystal. Crisp and Simpsons both make a 60L crystal and a 120L crystal that work nicely for this type of beer. I would go with 10 pounds of Optic (or Marris Otter if you can't find it) plus a pound of English 60L crystal or half a pound of English 120L crystal for your grain bill. I find that I like the malty-ness of the larger amount of lighter crystal a bit better.
For hops, if you can't get Northdown or Challenger, I would go with an ounce of EKGs at 60 and 15, and 2 ounces at 5. That should get you a solid 35 IBUs.