This suggests an alternative etymology, from the use of barm as just such a yeast.
From the en.wikipedia.org
I think you'll find in Wigan that is very close to the delicacy known as a pie barm.
From the guardian.co.uk
The dough is fermented for 24 hours using beer barm, frothy stuff from ale production.
From the telegraph.co.uk
We'd split and spread the barm cakes with butter for Mum, Dad and me.
From the guardian.co.uk
Patrick's Day, turn this into a barm brack, or yeast bread.
From the sacbee.com
We'd savour our barm cakes with conversation and large mugs of tea as we recovered from a Saturday night out.
From the guardian.co.uk
It was gradually replaced, first by the use of barm from beermaking, then, after the confirmation of germ theory by Louis Pasteur, by cultured yeasts.
From the en.wikipedia.org
For us this wasn't the traditional English with a plateful of eggs, sausage, bacon, black pudding and all the trimmings, but brisket barm cakes.
From the guardian.co.uk
Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.
From the en.wikipedia.org
More examples
Yeast: a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey
Barm is the foam, or scum, formed on the top of liquor (i.e. fermented alcoholic beverages such as beer or wine, or feedstock for hard liquor or industrial ethanol distillation) when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. ...
Barming is a civil parish in the Maidstone District of Kent, England. It lies to the west of Maidstone, the county town, and had a population of 2234 persons (2001 census). The eastern end of the parish is part of the built-up area of Maidstone, although the remainder is much more rural. ...
Bosom, lap
Liquid yeast that appears as froth on fermenting beer. (Can also be used as a verb meaning to pitch or add yeast.)
Yeast. More likely, the lees from a previous batch or the froth from a currently fermenting batch.
A British term for a yeast leaven. In brewing, the term "barm" refers to the foamy yeast residue from the fermentation of ale, then used to leaven bread (different strains of S. cerevisiae are used to ferment both bread and alcohol). ...
The foamy yeast that appears on the top of malt liquors as they ferment; ale barm was commonly used as the yeast element in breads and batters.
The yeasty foam that rises to the surface of fermenting malt liquors.