For steeped grains plus malt extract:
BJCP Best Bitter characteristic ingredients: Pale ale, amber, or crystal malts. Most contain sugar. May use a touch of caramel or dark malt for color adjustment. May use corn or wheat. English finishing hops are most traditional, but any hops are fair game; if American hops are used, a light touch is required. Characterful British yeast.
Best bitter vital statistics: OG: 1.040 - 1.048, IBUs: 25 - 40; FG: 1.008 - 1.012, SRM: 8 - 16, ABV: 3.8 - 4.6%
Use a British pale ale base malt base and about 5% light to dark crystal malt. Specialty grains that provide bready and nut-like character are possibilities, but use restraint. Other possibilities include small amounts (1% or less) of roasted malts and up to 10% torrified wheat. Sugars and invert sugars can be used up to about 10%.
British hops are recommended: Kent Goldings, Fuggle, Challenger, Target, etc. Most recipes have one or two late hop additions that add up to about 25% of the hops. The BJCP style guidelines state that emphasis should be on the bitter hop addition rather than late hops. Hop flavor is described as "moderate to low." The suggested BU/GU is .73.
Dry hopping can be pursued to replicate English cask conditioning. Use restaint because bittering is the primary hop quality.
British ale yeasts should be used. Posibilities are Wyeast 1968 (London ESB), White Labs wlp0002 (English Ale), Wyeast 1335 (British ale II), and similar ale strains. For dry yeast, try Fermentis S04, Lalbrew Nottingham, or Lalbrew London ESB.
Strong, G. (2017, September). British bitter: A proper pint. Brew Your Own, 28 - 32.
Zainasheff, J. (2014, November). Best bitter: An authentic English style. Brew Your Own, 23 - 27.