I have no knowledge of the expressions used by PB but I 'Googled' Plymouth Brethren and was surprised with all the details and mention of their Irish origins.
It appears very unlikely that we will ever establish Newbury 'Brethren' membership but 'Alfred Brown, Ministering Brother, Mr. Westalls Yard Newbury', was able to provide attendance numbers for 30 March 1851 [6 morning; 40 afternoon; and 6 evening] and indicate that the building was converted for use about 1846.