Your test was cancelled, or you need to move it
Cancellations run in both directions, and the rules are completely different depending on who is doing the cancelling. Knowing the line in advance is worth real money.
If DVSA cancels your test
It happens: examiner illness, bad weather, problems at the centre. You do not lose your fee, and you will be given a new date. If the weather looks bad on the morning, the safe assumption is that the test is on until DVSA tells you otherwise, and if in doubt ring the centre before setting out rather than deciding for yourself.
If they cancel at short notice, you can also claim reasonable out-of-pocket costs back, things like hiring your instructor's car for a test that never happened. The claim form and the exact conditions are on GOV.UK. Keep receipts.
The sting is that the new date can be far away and is not always convenient. A rearranged test is still a booked test, which means it can still be swapped: list it and let another learner's date fix what the cancellation broke.
If you need to cancel or move it
The number that matters is ten working days. Change or cancel with at least that much notice and your fee survives. Inside it, the fee is gone, and being ill or having something come up does not change that. Your booking confirmation email states your exact last free change date, and it is worth finding that email now and reading the date rather than discovering it too late.
One nuance for swappers: a swap is a change, so the same clock applies, counted to the earlier of the two tests. That is why every match on this site is checked against the ten day rule before you ever see it, and why listing early beats listing in a panic.
If you are ill on the day
Do not test unfit; an examiner can end a test that is not safe, and a test driven at half capacity is a fee spent on a likely fail. Inside ten working days the fee is lost either way, so the choice on a rough morning is between losing a fee and losing a fee plus an attempt. Take the smaller loss.
Waiting for a date you did not choose?
Somebody out there wants yours. List your test free, and we email you the moment a learner who fits appears. DVSA does the actual swap on the phone.