I think there are two main reasons why there aren't alot of farmhouses about -
1. Cost, the John Gittens buildings Mandy sells are very nice well detailed replica's, abeit a bit old fashioned more akin to a farm workers cottage than the main farmhouse and are quite strong money, as is the Brushwood farmhouse, which with some of Mandy's flemish or english style brick sheeting plus a bit of further modification could be a very nice house, but at over £100 to purchase in the first place it makes it expensive for us mere mortals on less than £18000/year
2. Geographical location, in Devon there are alot of the traditional devon longhouse's, however whilst working for different farmers and contractors in the past, I have seen different styles of house just in Devon, going further afield to different counties and across the borders to Scotland,Wales and Ireland there are probably more again still.
I guess what I'm trying to say is that if we're all honest, most of our model collecting/diorama building stems from real life experience and thus we want models to replicate this, but unfortunately for a manufacture to make replica's of every style of farmhouse similar to the areas we live in, wouldn't be economically viable or practical.
Speaking from a personal point of view I think it would have to retail for no more than £70 to be of interest