Funding ran out, can I leave I'm a PhD student in experimental biology in the UK. My funding grants me a monthly stipend for 3.5 years (including money for consumables/conferences etc). 

In my group, the standard practice is for people to work for 3.5 years and then write up (though actually different people have different funding, and the funding that the department provides directly does last 4 years). 

When I started, I was quite happy to stay for 4 years (the stipend is sufficiently generous that I've saved enough to cover me), but, relationships with my PI have deteriorated significantly. * At this stage, I've said that I wish to write up at the end of 3 years, and be  ready to viva by 3.5. This is something that she refuses to grant. 


  What should I do in this situation? I don't feel that anyone can be expected to work for free (theres no expectation that I should have saved money), and therefore feel that (1) she should either give me her blessing to write up, and all the support that goes with it, or (2) she should find additional funding for the final 0.5 years in order to force me to write up in the department. 


Note: the UK has a different funding model to the US, and I'd prefer to just hear answers relating to the UK funding situation (i'm well aware that students in the US have a far tougher time of things, both in terms of money, but also in terms of the very very long PhD's that some people must endure). 



*: We've had  a difficult relationship from the start, truthfully. She is fairly old and established at this stage, and has very little interest in discussing things, but rather dictates her wishes via emails from her secretary and then comes to the lab to complain when things don't work. This has slowed my work appreciably as we're constantly jumping around to keep her happy.
Unless UK universities differ significantly between each other, your supervisor will not have any role in assessing your final thesis - this will be done by the internal and external examiner.

Your supervisor will normally propose these based on personal experience - and I suspect in most cases the university will go along with those suggestions (at least that was my experience).
So a malicious supervisor can give you an examiner who will give you a hard time - while a friendly supervisor will suggest a friendly examiner.

So where am I going:
If the relationship with your supervisor has deteriorated to the level where you feel you would just quit - but are nevertheless motivated to complete your work, it would be highly advisable to seek some advice within the university (and possibly also if you aren't motivated to make sure you don't make a mistake). There should be a person responsible for dealing with such situations - I forgot what they are called...
Ideally, you could be given an administrative supervisor and submit the thesis under their "guidance" - they won't need to be an expert in the field, but should understand your work.

The key question in this case is whether the work is sufficient to be considered for a PhD, but only you or your supervisor (be it old or new) can determine that.

A side note on funding:
The university as an institution is unlikely to continue your funding, unless you are employed doing some work, say on a part time basis to carry on with your work. Where people are paid beyond the standard grant duration, this is done by the department - potentially the sponsor if there is an industry sponsor. I believe it is up to the department ow they organise this - i.e. whether they pay you to write up or whether they pay you to continue/complete your work before/after writing up but before defending the thesis.