OK then, then let me put it another way.
As a matter of law only, a contract is entered into at the point of sale when all the necessary terms have been agreed. In layman's terms, it means that the goods or services are (agreed to be) delivered in exchange for a sum of money. The analogy was drawn with a shop-keeper, and to take that a little further and expand upon it, the customer offers to buy some goods to which the shopkeeper agrees; the cost is also agreed and title to the goods is then transferred. Payment can be in legal tender (or is some other form agreed between the contracting parties - buttons, washers, sheep's heads, whatever). The normal course of business transactions carried out in this country, today, and accepted by all concerned, is that if more money is given at the point of sale, there is an implied condition to the contract that change will be given. This requirement can be excluded from the terms of the contract if there exists a notice saying (something like) "EXACT FARE ONLY" and that the notice is clearly visible to the contracting parties (and is, therefore, incorporated into the terms of the contract).
To address a specific point made earlier, and purported to be something that is not widely known, is that "if a customer gets on a bus (say) with a £20 for a £1.90 fare, what the customer is actually saying is "I am prepared to pay more than you are asking for."" I am sorry to have to say, again, but perhaps a little more diplomatically this time for fear of offending any sensitive readers, that this is absolute nonsense. Whoever heard of anyone going shopping and there being no chance whatsoever of getting change from any higher amount tendered? Because of the standard, well practised and fully understood terms of purchase and sales transactions, millions of which are carried out each hour of every day, it is a common form of trading that change will be given, so it simply does not stack up to say (with implied legal authority!) that "I am prepared to pay more."
The Royal Mint website does indeed carry information of what is legal tender, and the amounts of each denomination that can be passed off as legal tender. However, the parties to a contract can waive that restriction, and it is self evident that they do often. What the RM website doesn't do is give an exposition on what the law is in relation to sales and purchases contracts. That is for the legislature and the courts.
Car parking machines that state "NO CHANGE GIVEN" or other machines that state "EXACT FARE ONLY" are perfectly legal because they impose an additional contractual term before the point of sale. It is still open to the customer/purchaser to decline to be a party to the contract as he is not bound at that preliminary stage to the transaction. If he then chooses to offer more money, he is deemed to have accepted the contractual term by his conduct, and he is therefore bound by the terms (and cannot later whine about it or have any legal redress (assuming the reasonableness test in UCTA is not offended - other lawyers will know what I mean here)).
Finally, I have seen many, many times during my short association with this site that railway people can be quite dismissive of "facts" and "opinions" put forward by members of this forum who are not in the industry. Likewise, I have seen many examples of legal opinions being given by those who, seemingly, know someone whose brother's mate knows a lawyer but are, themselves not qualified and/or are learned in the law. I am sorry if I have offended anyone by what I have said (or, earlier, in the manner of what I said), but it would be good not to judge the messenger simply by the message that he gives. Whatever my personal views are of the law, the law is what it is whether you or I like it or not.