Thank you very helpful
Just another thought I was trying to do a route from Redhil to Warwick Avenue station but the site does not recognise the station Would the fare be the same as from Redhill to Paddington as they are so close?
Warwick Avenue is a London Underground station. London Underground works differently to the rest of the rail network (what's known as National Rail) in terms of functionality and ticketing, although it is still possible to buy one ticket that's valid on both.
Most booking sites won't have stations that are solely Underground stations in their list of options, so if you want to buy one ticket from your origin to your destination then you'll need to enter the London Zone that it's in - in the case of Warwick Avenue, that's Zone 2.
A ticket to London Underground Zones 1-2 would then be issued - and this is simply a single ticket that lets you travel to any Zone 1 or 2 Underground station.
Alternatively, you could get what's known as a Travelcard. A Travelcard is a kind of ticket that is valid for unlimited use on (almost) all services within the Zones you select. You can get a Travelcard for varying durations - you can get a Day Travelcard that's valid for just one day, you can get a weekly one and so on. You can also get them for different zones, and have one ticket that serves as both your normal rail ticket and a Travelcard. When you buy a ticket to "London Terminals" you'll usually be given the option of asking for a Day Travelcard. In most cases the only option is for a Zones 1-6 one.
The other alternative is to use an Oyster card, or a contactless debit/credit card (or contactless payment device, e.g. Apple/Google Pay), and to use this for the section on the Underground. You'd then just buy your rail ticket to London Terminals. If you're only making one return journey that day, this is probably going to be the cheapest option.