FLight from Boston,US lands in which terminal

It should be noted in your travel booking - check the email with your e-ticket receipt.

I expect your flight will arrive at Heathrow Terminal 3. Check your actual information to confirm.

My regular flight was BA from Boston to London Heathrow...precovid. My flight ALWAYS landed at Terminal 5. However, I think most car services will look this up on the day or you could email them the information confirmation when you print your boarding pass at worst.

https://www.google.com/search?q=british+airways+boston+terminal

Terminal 5

I think if you are booking a car service they will monitor your flight arrival.