A few things:
Use Maris Otter and EKG as others have said. Dry Hopping can really do alot of good with a bitter. Also, I personally love to use biscuit malt in british brews. In my PacGem Special Bitter, I used half a pound of biscuit, and I wish I'd used more. Also, just to make sure you're getting an authentic style, be sure to drop in some Burton Water Salts...unless you've already analyzed your water and it's close to Burton-on-Trent's profile.
And definitely use a british yeast. I like Wyeast's London ESB, which lends fruity flavors to the brew and flocculates with crystal clarity in several days...in the PGSB, I used WLP005 because it's what I had on hand, and it worked very nicely.