Unless you're planning to drive about on your career break, you'll need to make arrangements for your car.
Advantages of selling your car:
Using a dealer is usually the quickest and easiest way to sell your car. Drive it round to a few to get a decent price.
If you're selling your car to someone you know, ask for the money as a banker's cheque unless you trust them completely.
When you sell your car, remember to:
Lending your car to a friend is an option if you're not going away for very long, or if you're particularly attached to it (the car, not the friend).
Once you've found someone who wants to borrow your car (and who doesn't drive like a maniac):
Note: As the owner of the car, you will need to be insured to drive it as well as your friend (even if you're out of the country). If you're lending the car to your partner, you may be able to take yourself off the insurance.
Leaving your car sitting about is not recommended, as that can result in serious damage. It needs to be driven weekly. Also, as well as the costs of storage, your car will depreciate (about 10% over six months, even more in winter).
However, some career breakers can't bear to do anything else with their car, so they put it into storage. There are storage facilities that leave your car outside to gradually fall apart, so you will have to get it sorted out on your return (the brake pads will seize up amongst other things). You will have to declare SORN (telling the DVLA that your car is off the road) and notify your insurance company. This option is around £65 a month.
Professional car storage is more money but they will take proper care of your car. Usually, there are a range of options that you can choose, eg driving it, cleaning, polishing etc. The garage can also sort out your MOT, SORN (declaring your car off the road) and re-taxing when you come back. Rates start from around £15 a week for the basic storage; extras start at around £4 a week.