How often do PhDs take longer than three years? I'm an EU student, so I'll have to support myself. At the moment I have £15500 at my disposal (working/savings/study loan), and I may be able to convince my parents to lend me another £7000. This should be enough for three years I reckon, but what happens if I'm not able to submit my PhD in time? Would it be at all possible to write the thesis in my home country (a lot cheaper), once the experiments are done?

As long as you can complete your lab work within three years then you will probably be able to write up from your home country. It is perfectly possible to finish lab work in two and a half years and then to write up in three months. However you'll have to be very focussed to do this (and have a litle bit of luck).