or back sweeten it with a non fermentable to get it to style!!
You can get pretty much any attenuation you like if you mash for it. I tend to leave my mash overnight so I always get good attenuation. Adding enzymes is a very blunt tool and will dry out the beer completely and I can't think of any British real ales that use a diastaticus variant yeast.
People have to remember that British bitter has its origins in the original IPAs from Burton (Staffordshire), which had to be fermented right out (by diastatic
Saccharomyces and/or Brett) before they travelled to India, as any residual sugar would allow fermentation to restart in the tropics, causing barrel bombs. And then people (and brewers!) decided they liked that flavour, because it encourages you to drink more, whereas sugar triggers satiety.
British beer is all about sessionability/drinkability and just generally about balance, which is why it's so depressing that so many US interpretations are flabby sweet messes. As the BJCP say,
ordinary/
best bitter has "Low to medium maltiness with a dry finish". I get that USians tend to visit the Thames Valley which is at one end of the spectrum - the
Fuller's partigyle uses 7.2% light crystal which is plenty for my taste, but tourists tend not to visit northern parts where the Burton influence means members of the saison family are still quite common - which include WLP026 ("from Staffordshire", allegedly Marstons), WLP037 Yorkshire Square and WLP038 Manchester. One can assume there was something diastatic in the classic "Tadcaster" yeast blend that Boddies were using to get apparent attenuations
pushing 92% in the 1970s.
This idea that typical British bitters are using Windsor-type yeasts to get <70% attenuation is just so wrong - Windsor was isolated from a multistrain that also included the precursor of Nottingham, which
goes up to 84% on the Lallemand standard wort. Northern bitter at least, is deceptively simple, taut and precise - if it was music it would be that opening clarinet solo in
Rhapsody in Blue. Too many people try to turn it into prog rock.