Maris otter is a must for bitters (or halcyon or optic, etc.). Also, british crystals make a more authentic bitter than american crystal malts. If you want to go simple, maris otter with 5% british crystal makes a great bitter. You can expand from there. And any combination of EKG, fuggles, northdown, styrian goldings, or challenger hops makes good british beer.
I'm too lazy and have too many beers going to keep updating this!