Travel to Dover from Paris

Train : Paris Gare de Nord > London St Pancras

Train : London St Pancras > Dover Priory ( https://www.nationalrail.co.uk/ look for Advance tickets)

is the easiest and quickest

The simple answer would have been ferry. However none of the ferry companies from Calais to Dover are now accepting foot passengers. And although Flix Bus use the ferry and call at Dover they won't set you down there.
So I think the simplest answer is to take Eurostar to London St Pancras International (Eurostar Terminal), then change within St Pancras International Station to the South Eastern trains platforms and catch a High Speed service back to Dover. That takes 65 minutes.
There is very clear signage between platforms.
Before Covid and Brexit you could have changed at Ashford (International) but that station remains closed to Eurostar.
You can usually get an advance fare for between £11.30 and £17.50 for the St Pancras to Dover leg.
As we seem to have started to quote station codes on this forum (which no one among the general public uses in real life) the St Pancras code to use is STP, not SPX.
EDITED- I have been researching why St Pancras has two station codes in the public booking systems and can't come up with an answer. Looking at the detailed departure boards shows that both all domestic and all international trains use STP (London St Pancras International, in booking engines and internally in the industry), so SPX (London St Pancras (Intl) in booking engines seems to an out of use code, which is why it produces no results if used in public booking engines.

I use the station code when i know it when booking tickets
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