Originally Posted by rcspeirs

The tube ride from Heathrow to central London? Yes, the system has zones but you don't need to know or care. Why don't you care?
If you buy a cash (paper) ticket, which is the most expensive option, it's a flat fare.
If you use oyster or contactless (cheapest method) the system logs you in and logs you out then calculates the appropriate fare - you don't need to know or care what zones.
Don't try to buy a through ticket from Heathrow to Kent. It's way cheaper to buy the tube ride and the national rail section separately. (I could try and explain the vagaries of uk rail prices but you'd lose the will to live)