I believe my starting point for water minerals was Martin’s brown malty profile, though looking things back over I didn’t approximate it real closely. Here’s my mineralization in ppm: Ca = 48 / Mg = 6 / Na = 30 / SO4 = 62 / Cl = 72 / HCO3 = 55. It there possibly a mineral contribution responsible?
That's an under-mineralised profile for any kind of British beer. To give one example, Murphys, one of the main labs in the UK suggest
for mild (their column headings are wonky) :
Ca= 100 / Mg = 10 / Na = / SO4 = 150 / Cl = 200 / HCO3 = 50
As you can see, they suggest at least 100ppm calcium and 200ppm chloride for all British styles, and 100-400ppm sulphate depending on style. US ideas on water chemistry for British styles are a bit strange.
But some British strains produce a bit of lactic, S-04 is notorious for it and WLP006 is a pretty close relative of S-04, so I'd look to the yeast.
The other thing is to look at other sources of acidity, such as carbonation - Murphys suggest a finishing pH for cask beer
of 3.7-4.1, US brewers seem to aim a bit higher and then let carbonic acid from their higher levels of carbonation do some of the work of reducing the pH.
So what's your final pH, and what carbonation are you using?