Travel from London Zone 2 to Heathrow, zone 6 using a Travelcard.

Heathrow has special rates.

Zone 2 to Heathrow the rates are 3.30 GBP during peak hours and 1.90 off peak.

I believe that if your travel card is on an Oyster card and you have put money on the card, it would work. Other wise if you just swipe at Heathrow it will assume you are getting on at Heathrow.

I have never used a travelcard. I just pay the 1.90 GBP with my Oyster card.

National Rail stations now issue London zonal Travelcards on Oyster Cards so there is now no difference with respect to TfL as you wouldn't be using a separate card.

Thanks, I have an Oyster Card but am planning to buy a 7-day Travelcard from a National Rail Station so I can qualify for 2For1 admission to many attractions. I will be taking the train to London from Cambridge and flying from Heathrow 6-days later. According to information on the National Rail site, persons can qualify for 2For1 either with a train ticket for that day or by using a Travelcard issued by National Rail with the National Rail logo and my photo, as long as it is possible to reach the attraction(s) using transportation covered by the Travelcard. The 2For1 savings would justify obtaining the separate Travelcard, rather than loading it on my Oyster Card, but it sounds as if doing so would prevent me from using a combination of my Travelcard and my Oyster Card to pay for travel to Heathrow. I can afford to separately pay the 1.90 with my Oyster Card though since I'll be traveling from zone 2 on a Sunday.

You would have to leave Hammersmith station with the Travelcard and re-enter with the Oyster card.

Since June National Rail stations within London no longer issue paper zonal 7-day travel cards so you will need to change your plans.

I don't know the source of your information that National Rail no longer issues paper Travelcards for London. According to the National Rail website on 9/17/2022, although National Rail can load a Travelcard onto an Oyster Card, a Travelcard paper ticket can also be issued at any National Rail ticket office as long as the individual supplies a passport sized photo. Am I missing something?

You have probably found an obsolete page on that website, it is riddled with them.

For example, here is information from one train company, SWR, that they won't be issuing paper travelcards after 1 June: https://www.southwesternrailway.com/train-tickets/ticket-types/london-travelcards/important-changes-travelcard-seasons

And here is an up-to-date National Rail page:

In particular:

PLEASE NOTE: within the London Fare Zones Travelcard Seasons can only be issued on Oyster or TOC Smartcard. They are no longer available as paper tickets

Thanks for the information. I did more research and discovered that the 2For1 offers are still valid with a 7-day Travelcard if that Travelcard is loaded onto a Thameslink (or possibly other regional rail co) smartcard. The smartcards cannot be mailed to a U.S. address but can be picked up at rail stations in the U.K. We'll just have to wait until we get there to obtain smartcards and load on the Travelcards.

Yes you can load onto TOC smartcards as I indicated above but these are not always read properly by TfL infrastructure in my experience (I have one from GWR). Maybe this is getting more reliable. You would still have the problem of extension tickets as in your original question.