I've always read that a PhD is the equivalent as working in the industry. Saying that, could you negotiate your salary before you start the PhD?
Or is every PhD person doomed to have as minimum stipend as possible?
You will have very little room to negotiate your salary / stipend. Whether there is any room at all depends on who is providing the funding: If the funding comes from an industrial source then you may be able to negotiate a few extra k, however if the funding comes from a non-industrial funding body (e.g. EPSRC / EU etc) then there is unlikely to be any chance for negotiation.
Remember that your stipend will be untaxed, and you will be exempt from expenses like council tax, national insurance or student loan repayments. The typical ~14k PA for a PhD stipend seems very low at first glance, but once you take into account all the benefits it looks a lot more like a £25k PA salary.
... As an aside, there is generally *very little* room to negotiate salaries in academia full stop - unless you're a professor. Salary scales are generally pretty rigid, and you will progress at a fairly constant and steady rate. You will get a modest bump with every 'major' promotion (i.e. post-doc to lecturer, or within the three grades of lecturer), but aside from that there is very little that can be done.
This can cause no end of staffing problems... In industry there is always the option to increase the salary to attract better talent, but post-doctoral research positions have a salary set by the University pay-scale, with no room for maneuver. Without the means to adjust the salary, things can move very slowly. Finding staff for post-doc positions, particularly 6 or 12-month positions, can be very difficult. If we could bump up the salary a little then I'm sure it would be easier to attract talent, but we just don't have that option - even when there's ample money left in the budget for salaries.