I've made JB Bitter up twice with 1Kg med spraymalt, it works very well indeed - it makes a lovely malty bitter which I like. I reckon DaaB's recommendation to add a few Goldings would cap it off a treat.
Interestingly, I made up both brews exactly the same and fermented under exactly the same conditions, except for the yeasts - In one I used Safale 04, and in the other Munton's Premium Gold.
The Munton's fermented down to a slightly lower FG, but I definately preferred the flavour of the one brewed with Safale. I still find it amazing that a change of yeast can make such a discernable difference.