This reminds me of the Swiss system, where there's a notion of the "half fare card" which is conventionally supplied by companies in Switzerland to their workers. 50% of the ticket price is essentially a "tourist tax", which is unavoidable by buying a different kind of ticket (like an Oyster card) at the station. I think it's pretty smart.
There is nothing to stop visitors from using either contactless card payments, Android/Apple pay, or an Oyster card. Oyster cards are easily purchased from all stations, especially places many people arrive like Heathrow Airport. They're promoted on websites aimed at tourists like visitlondon.com, and covered in every guidebook to the city. The £5 deposit and any remaining balance is refundable at many machines, and doesn't expire.
You can, but it will cost ~$120-150 if I recall correctly, so unless you're making several trips it's rarely worth it.
They do have a 'secret' ticket called a Swiss Transfer ticket which will take you from any airport to any train station and back for a much cheaper price than a normal train ticket. However you cannot buy it in Switzerland, but have to order online before your departure.
