Heathrow to Broadstairs and back

Train.

Piccadilly Line tube to Kings Cross St Pancras- journey time 1 hour.

Switch to South Eastern High Speed departing St Pancras at 20 minutes past each hour (journey 1:44) or 37 minutes past (journey 1:25)

Return at 25 minutes past each hour (the slower train) or 8 minutes past each hour (the faster train)

Buy South Eastern Advance Tickets in advance for the best prices- should be around £15.50 each way.

So you spend around 3 hours travelling in each direction.

There are National Express buses (change at London Victoria but they take close to 5 hours each way.

Another option would be the Elisabeth line till Stratford, and Southeastern High speed from there.

IF you took the Elizabeth Line route (which is no faster) you would need to check that you were on a train that was headed for Shenfield, NOT one headed for Abbey Wood.
You don't say which Terminal you are starting from but from Terminal 4 you would have to change trains at Liverpool Street (or any of the Central London stations) and from the other terminals there is only a train every 30 minutes.

The tube is far more frequent, so is the better route in most circumstances.

I'm curious about the Broadstairs attraction. Yours is the second in a week after years of nobody mentioning it. What's going on there?

Hello Nigel
I am the other Broadstairs poster LOL and was surprised to see another post as well! I’m curious about this OP hc we’re headed to BSR this summer to see some different sites including Rochester / Chatham (fans of musician/artist Billy Childish); Canterbury (punt ride and cathedral); plus Broadstairs itself.

Broadstairs bc we saw a 2018 movie filmed there, and it looks like a neat area to explore after a busy week in London. The movie is Juliet, Naked (no nudity! It’s a song title). If you ever geeked out about rock musicians, or love a good rom com, it’s a charming movie based on book by Nick Hornsby (also wrote, About a Boy) .

“(Rose Byrne) is stuck in a long-term relationship with (Chris O'Dowd) - an obsessive fan of obscure rocker Tucker (Ethan Hawke). When the acoustic demo of Tucker's hit record from 25 years ago surfaces, its release leads to a life-changing encounter with the elusive rocker himself. JULIET, NAKED is a comic account of life's second chances.”

A Dan Levy movie, “Good Grief” was also filmed near there (the end of the movie) in Margate (?) at an art gallery.

For us it’s fun to see these places in “real life” and then rewatch when we’re back home. A nice aside from some of the bigger sites :)

Enjoy your trip!