I've seen (magnetic) compasses being used in planes, which to my knowledge fall in the category "large metal objects".
The deviation incurred by the metal is corrected using small screws / magnets, equal to the deviation correction on a more expensive compass. So i haven't looked for car-compasses which can be corrected, but they should be around. However expect to pay more than 2 quid for one, as a decent handheld compass costs around 20 pounds.
Hope this helps.
The best planning is to go!