Yeah that seems a sweet idea. I believe to classify as a Manor House a house needs to have over 6 (ish) bedrooms and be at least 5,000 square feet. In addition to that you’re meant to have lawn and garden too. A a cottage would be much smaller and easier to design.
This is just an idea but do you think that “cottages” should only give 5 population space, have less hit points but have double the build limit. Their cost could be 110 wood and they would still produce a settler on completion. This would allow British players to continue booming well into the Industrial Age but make them constantly wood dependant.
Maybe to balance this you remove all villager Home City shipments from the British deck so that they have to send more Cottage (manor) related ones instead…