My thoughts are we can release about £70k equity in our house, possibly £75k, we have a bit of savings as well that we can use. I would like to use this to purchase the land and have roughly a 10% contingency, then get a self build mortgage of around £200k
If you are going to plan budget of £275k then I can almost guarantee it will cost over £300k by the time you have finished. Have you looked at what is available ready built in that budget in your desired area. Any figures you have got from the internet may be unrealistic in your location with the problems of materials prices rising that Brexit seem to be causing.