Hello,
I have managed to link Isaiah to Elias Benjamin (1820-1897); they were partners in an outfitters business in Shadwell (see attached PDF file).
The PO directory of 1844 still listed Isaiah as an outfitter of 47 High Street, Shadwell. By 1849, he was a clothier and oilman at the same address.
Abraham's father appears to have been the son of an Elias, who was himself the son of Asher 'the cakeman'. An early burial register recorded that Elias was buried on 27 Dec 1801 in Spitalfields. His wife, unnamed, was buried on 4 Apr 1808. I shall check where the cemetery might have been.
I would tentatively speculate that Asher cakeman was the son of an Isaiah, perhaps Isaiah son of Benjamin (i.e. Yeshiyahu ben Binyamin).
Abraham's older brother, Asher (named after the deceased grandfather), concluded a seven-year apprenticeship under watchmaker Lewis Casper (costing £12) on 10 Jun 1803.
Justin