Traveling from Cornwall (St. Erth) to Heathrow

I use the website https://www.nationalrail.co.uk/ for UK trains. After selecting a departure, you'll be switched to the website of the operating rail company. Enter your departure point as St Erth (with no period) or it won't work. You'll be on a Great Western train out of St Erth.

The trip doesn't take as long as I feared--under 6 hours. However, I would not want to wake up in Cornwall on the morning of my departure (on what I'm guessing is a transatlantic flight). A lot could go wrong. I have no idea what sort of reputation Great Western has for reliability, for one thing. My philosophy is to spend my last night close enough to the departure airport that an emergency taxi would be an option; a taxi from Cornwall to Heathrow would be exorbitant, and ViaMichelin estimates the driving time at almost 5 hours.

Should you decide to spend your last night near Heathrow, I can recommend the Premier Inn at Terminal 4. It usually has really good rates. It's connected to the terminal by an enclosed walkway, so all you need to do is walk to T4 and take the Underground to the T2/T3 stop. There may be hotels located right at T2/T3, but I think you'll find they cost considerably more.

St Erith to Reading book with https://www.gwr.com/ ( or https://www.lner.co.uk/ if GWR doesn't like US credit cards)

Reading to Heathrow https://www.railair.com/timetables - take the coach to Heathrow

Unless you are travelling on a Saturday night there is also the Riviera Sleeper Train from St Erth to Paddington. It is far cheaper than the Scottish sleeper, and most people think is a far superior product. That could be a great way to end your vacation.
You can stay on at Paddington after the train arrives until 7am, then freshen up in the 1st class lounge, then backtrack to LHR.
It's the one time I would say t do Paddington, then backtrack.
Use the GWR website/app.
There are also 2 National Express coaches a day- one at 0830, change at Plymouth, arrive LHR at 1705, or a direct overnight one- leave at 2135, arrive LHR 0530. They leave from the Railway Station entrance.

On the day trains you can book a through rail ticket from St Erth to Heathrow, changing at Reading and Hayes and Harlington, for as little as £44, as an advance fare (non railcard fare- £29 with a railcard).

I saw some great fares when I spot-checked the schedule before responding. I was very surprised it could be so inexpensive. Of course, I was looking at dates in April, which I assume isn't exactly peak season in Cornwall.

Thanks for the info.