I’m not sure I see the ‘middle-class’ thing (neither sounds particularly middle-class to me). Sally’s parents were tenant farmers in Leicestershire, and Moth seems to have had a father in the building business.
I don’t think we know anything much about their level of education, do we? They met at college, but it’s not clear whether either actually finished whatever course of studies they were on (lots of references to skipping classes). Some interview credits Moth with a botany degree in the past, but if he already had a degree, presumably he wouldn’t have been eligible for the student loan for the degree he’s doing during TWS? (And wasn’t that an botany degree?) And Sally refers several times to being ‘unqualified’ for jobs. Would a bookkeeper for a small business when she did it need formal qualifications?
And isn’t the whole point of TSP that they weren’t homeowners, or even renters, which is why people were shocked they owned property in France? I assume they are now, or that, if they aren’t, it’s a choice.