I doubt these houses had servants, they are former farm-workers' cottages - their ovens would have been used by the family.
Re. lack of sorching/soot, I too wondered about that. You might think that the ovens had never been used. However, I spoke to the guy who uncovered our oven and he said he removed a huge amount of old ash (wheel-barrows full) from the ash pit directly below i.e. it had been well used. Perhaps the wood-ash keeps the bricks clean (I used white ash to clean the glass in my woodburner - surprisingly effective), perhaps they scrubbed or swabbed the inside of the oven, or perhaps they had some other trick?
