Enshrined in UK law is the principle that a retailer is not obliged to sell you anything, in essence the potential customer makes an offer to purchase, and provided the seller is not using any illegal prejudice they can choose if they want to sell to you or not. It is up to both the seller and customer to agree the terms of the transaction.
These days the process has largely been preempted (simplified?) by retailers showing the advised retail price, and having prewritten T&C's., but in principle there is nothing to stop customers trying to renegotiate price and conditions.
As it stands, there is no requirement for a company to have pre written T&C's for their sales, and consequently, except for the customers statutory rights, there is no regulatory control of what they can include.
In a sellers market its likely the seller can be quite resolute in how they tie the contract up , but in a buyers market the seller may need to consider the customers wishes a bit more to prevent loosing sales to competitors.
As for what is actually written in the T*&C's, most companies these will have used a legal advisor to produce them, but as they do not have to be formally approved by any regulatory body before they are issued, they can include almost anything.
Again in the UK it is generally accepted that when a customer and retailer agree a contract,it's governed by the agreed terms, and the law will not intervene, unless one or other of the parties asserts there has been a breach of contract, or one party asserts one or more term is unfair,and of course in these modern times if one or more of the consumers protective rights have been breached.
T&C's may not be backed by specific legislation, they do form part of a contract in common law terms, and are likely to be upheld, unless they are found to be unfair or in breach of specific area's where legislation has been enacted.
This means customers cannot ride rough shod over T&C's they should be understood before entering into a contract.