... my opinion is that once the key is discovered Yarmouth will be understood as either a complete red herring or a simple mis understanding by the enumerator.

Middlesex seems to be the common thread in his young days, Surrey / Middlesex and "london" boundaries would meet in the area of Battersea / Clapham.

However Bethnal Green is across London - not far but still distinct to avoid confusion with Battersea.

Now if Battersea turns out to be Bermondsey, then Walworth comes to mind.

Walworth (spoken in a south London accent) to a Gloucestershire enumerator, would he or she know about Walworth ? - would they assume he had said Yarmouth - just guesses at the moment.

Now the other consideration is the Whitecliff Pottery. There happens to be a Pottery tradition in Lambeth / Walworth South London. In fact Doulton of Royal Doulton fame is associated with the Lambeth Pottery, so is Yarmouth a mis-understanding of Lambeth ?? I can almost hear an accented Lambeth become Yarmouth.

At the end of the day its just an idea....

<update>
In Lambeth's Cemetry Database
Grave number: 5230-111
Grave Owner: Harriett Jane Blanch
Address: 23, Merrick Square, Dover Road, Surrey
Original Burial: 1857
Last Burial: 1881
Deceased:
Gustavus William Blanch buried in 1857
Harriett Jane Blanch buried in 1881