We are traveling from Paris to York, UK. We would like to spend a couple days in London. Can we get one ticket that covers both the train Paris/York trip as well as the downtown London Tube & bus travel? Or do we need separate tickets?
You need a ticket for the Eurostar Paris to London.
You need a ticket for London to York.
Within London, you use a contactless payment method (phone, bank card, Oyster) rather than tickets.
As noted, while in London you'll be using your credit card to tap and pay on the Tube and on busses. There's a max you can be charged every day. Unless you want an Oyster card or want to use an Apple Watch or similar gadgetry.
As the others have mentioned, you would travel Paris to London on the Eurostar -- https://www.eurostar.com/us-en
Eurostar arrives at St. Pancras Station.
Travel from London to York is mainly by LNER -- https://www.eurostar.com/us-en
Though Lumo also operates some trains on the route -- https://www.lumo.co.uk/
Trains to York depart from Kings Cross Station, which happens to be just across the street from St Pancras.
Lumo trains don’t stop at York.
Just FYI, Transport for London operates London's Tube and bus systems. Info here: https://tfl.gov.uk/.
Fare and payment info: https://tfl.gov.uk/fares/.
Contactless payment info: https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/pay-as-you-go/contactless-and-mobile-pay-as-you-go?intcmp=55539
For Tube journeys you tap in when you enter the station and tap out when you get to your destination. For buses, you just tap when you get on the bus, and you don't when you get off.
An Oyster card isn't necessary for contactless payment, but I have one and prefer using it. If I drop it in the rush of a Tube station, it's not as disastrous as if I dropped my credit card. Likewise, I'd prefer not to use my phone for the same reasons. It's just a matter of personal preference.
==>"Lumo trains don’t stop at York."<==
Duh! Thank you. Right Lumo serves a rather limited number of stations.