Don't get diversification confused with quantity. I'd say Liverpool is less diverse than Doncaster despite it being a lot busier.
Without doubt for me as the most diverse airport in the country is Manchester. Certainly in terms of passenger services.
Manchester sees regional/domestic, short haul, long haul, low-cost, full service/flag carriers and holiday charter, all to a significant level.
Plus Manchester is pretty much the only airport to see just about every major airline operating to/from the UK. For instance BA, Virgin, EasyJet, Ryanair, Jet2, Monarch, Thomson, Thomas Cook, Flybe and BMI Regional all operate from Manchester and is the only airport in the UK where all of these airlines operate.