The standard PhD takes an average of four years to complete in the UK. You may live at or near to your university for most of this time, and may spend a significant proportion of the time at the university in a laboratory or office provided for your use.
Often, the equipment needed for the research will be provided by the university and one or both supervisors will be staff based in the same department. There will also be technicians, secretaries and many other support staff who will assist you should you need it. You will have access to libraries and will be able to participate and collaborate with research groups and experts in their university. From the perspective of the university, all of these facilities cost money. So even if you never attend a single lecture again, your PhD fees are needed to cover the cost of equipment, space and time of the staff who assist you.
But you do not have to study full time. If you are working and would prefer to do your PhD part time, then most universities will allow you to register as a part-time student. This may have significant advantages: because you may rarely use the university facilities (indeed, you may not be given an office or space in a lab), the fees are usually a fraction of the cost for full-time students. You will also have much more time to complete your doctorate - part-time PhDs are often allowed five or six years instead of three.
But there is a downside. Most universities prefer not to take on too many part-time PhD students because statistically, the success rate is very low. With reduced access to the university facilities and staff, distractions from work, and so many years in which to lose motivation and direction, most part-time students give up and never achieve their PhDs.
There are several other options, some better, some worse. One interesting variation is offered by the Open University. If you are living in the UK, you can register to become an `external part-time student', and find yourself a first supervisor at the university whom you would see every few months. Your research is then performed at a collaborating institute or university (at your own cost), which could be one of many in the UK.
A typical student might be a researcher in a UK lab who could use the facilities to do his or her own research for some of the time without extra cost. Because the Open University does not host the student, the fees are tiny (£119 per quarter in 2005) and you are only charged the home student rate if you are living in the UK - a significant advantage if you are not a UK or EU citizen. But of course, as a part-time foreign student you must have a work visa to stay in the UK, and a steady job. You cannot use a part-time studentship to keep a student visa. And even with such a good deal, the Open University still finds that many part-time students either give up, or register to become full time after a while.