The main thing to realise about tax rebates is that they’re not some magic windfall or dodgy way of “cheating the system”. The money you’re claiming back is yours by law. While the PAYE system does a decent job of working out the tax you owe on your basic salary, it falls flat on its face when it comes to critical expenses like travel to temporary workplaces. Unless you let HMRC know exactly what you’re spending just to keep doing your job, the taxman has no way of giving you the tax relief you’re owed. HMRC will never deliberately rip you off, because it only wants the exact tax you owe and not one penny more.
The tax rebate system is a way of levelling the playing field with HMRC, giving it the information it needs to get your tax calculations right. Of course, since the taxman’s not a mind reader, he can’t always predict what your expenses will be. Every time you reach into your own pocket to pay for the essentials of your work, a chunk of the price you’re paying is tax. That chunk is what you’re claiming back in your refund, and it’s why you have to keep good records of your spending top get back everything you’re owed.