A wealth of consumer protection legislation exists in the UK for the benefit of anyone buying goods and services. Big companies are only too aware of these laws, whereas the detail at least remains a mystery to most consumers. There is no reason to worry about this. If something is wrong, common sense will tell you that it is wrong.
There is no reason to not complain just because you are unsure about your legal rights. On the contrary, you will often find that reputable companies will compensate you to an extent that far outstrips the legal obligation to do so. Why? Because they value loyal customers and are constantly striving to improve the quality of their products.