Should the PhD proposal be discussed and adjusted by supervisor and student before formal application? Many social science departments in EU/UK recommend getting in touch with potential supervisors first before submitting a formal application which involves a proposal. Should the applicant approach them with a proposal already (very) specifically written? Or just broadly stating research interests and the intended project scope for the next few years? 

Is the PhD proposal supposed to be well-prepared before initial contact with potential supervisors, or agreed and finalized after consultation and discussion to fit both parties' interests? 

I understand that academics get many emails from potential students, so I wouldn't imagine they'd like to read through everyone's proposals. This question is informed by other posts like this, this, and this - but most relevant discussions I see here on SE are either quite specific to the science/engineering community or North American system, and I'd like to hear about what is the expected pre-application communication in UK and EU, and/or in other disciplines, especially in social science.
You will probably join an existing work group, and work on the projects of the advisors. You will get some job that (somewhat) aligns with your interests (and competence), as asking for stuff you don't like and aren't competent in is just wasting a (valuable) position. Other than that, your thesis will be directed by their pressing interests, not yours. Laying out a detailed proposal as first contact is just a waste of everybody's time. You will work it out with them, eventually.