I wrote a book on the DWP Work Programme, from the point of view of one participant. I doubt that many conservative publishers would want to touch it - because it describes experience of being there and life in general the point of view of the people who were sent on it. Are there any more socialist-orientated publishes who might be interested in it?
If there is a profit in your book a publisher will publish it. Political content is not as important as the money it will make.
I am a keen Liberal Democrat supporter and no doubt that shows in what I write, yet I have never found any publisher or editor mention it in their dealings with me. It is content that matters.
The trouble with ‘homing in’ on your own kind is that you are preaching to the converted when what you ought to be looking for are new converts. In any case, Labour, Liberal Democrat, SNP, Conservative parties (and others), all have publishing wings; I suggest you try them. If they refuse, do a re-write and try again.
Please let us know how you get on.