Just found I had another directory - Pigots 1830. There's a second Thomas Selway in there, also in Bath, but he's a commissioner for oaths... 34 Kingsmead St Bath. Unlikely I would think?
Added:
Having checked back again through the thread and realised that Harriet is significant, I thought I'd better list out the trades directories entries for her in more detail

Robson 1839 - Bath - Selway, Harriett
milliner and straw hat maker - 18 Green Street
She also appears at that address under the 'Milliners and dressmakers' heading and also, (spelled Harriette) under 'Straw Hat Makers'.
Pigot's 1842 - Bath - Selway, Harriet & Eliza, milliners, 18 Green Street Interesting that they are also in the pages given to milliners, but they aren't listed under the Straw Hat Makers section.
Post Office 1852 - Bath - Selway, Misses H & E, dressmakers, 17 Broad St