I think that UK PhD students usually aim to finish in 3 years, but don't always manage it. However, it seems to be the actual writing up of the thesis that takes the extra time, rather than the research element. This is not helped by the course guidelines though - my masters is only 1 year long, but I am allowed an extra 2 years to write up and submit the thesis!
I think the problem regarding writing skills over here, especially in the sciences, is due to the fact that we've never been taught how to write essays and papers properly - or at least I haven't.
Because of the GCSE/A-level system here, most students stop taking English classes at 16 and specialise in other subjects (I took A-levels in Physics, Maths and Geography and have not taken an English class since 1999). The students taking A-level English study sentence structure and essay writing in school, but no one else seems to learn these skills.
In my 4 years at Exeter, I did not have to write a single essay or paper, except for my final dissertation (which was basically 30 pages of maths and only about 15 pages of actual writing) and was not given any guidance at all about how to write good papers. However, I have just had to write a research proposal for my masters and had to be reminded of the correct way to cite references, figures and equations along with paragraphs etc., because I've never had to do it properly before.
An example of how well (or not) proper English is taught here these days... until 4 weeks ago, when I was studying for the GRE exam, I could not tell you the difference between a verb, noun or an adjective - I don't ever remember being officially taught them in school!