If you really believe that your test was unfair - it's within your rights to appeal but you must do so within 6 months.
However, although everyone is disappointed if they fail - remember that sometimes the pressure of the test can make events feel different than they actually were.
The examiner will give you a short debrief on the main mistakes made during your test - listen carefully. Learn from the debrief so you can practice those areas of your driving that need improving.
There is an explanation of the marking system on the back of the driving test report.