Consecration is performed by a particular religion/denomination. In an public cemetery in England, this will usually be Church of England or Roman Catholic. Whilst I haven't explored it all, I haven't seen an area of obviously Jewish graves at Plumstead (Hebrew inscriptions, Stars of David, and pebbles rather than flowers), so it's most likely its consecrated plots are C of E and thus inapplicable to your Jewish ancestors. I think I've seen cemetery tariffs where unconsecrated graves were cheaper than consecrated, which would have been another factor for many people.
"Why were my London ancestors buried so far from home?" is an FAQ to which I've never seen a definitive answer. Well before the 1930s, cemeteries within the built-up areas of London were pretty well full, so most burials took place in the large cemeteries on the outskirts. Which one probably depended on where there was a timeslot available or maybe who was offering the best prices to the undertaker used. For "pauper" burials "on the parish", the Council would get the best price contract without caring too much about location.
As John is buried with 8 other people and Ellen with 6, there are probably "common" graves. As such they are less likely to be marked. Many sections at Plumstead have areas of bare grass, but there are scattered less-than-knee-height stones frequently used for such graves, so don't give up yet. The cemetery should have records of headstones as they usually have to be approved.
