Is this actually a day trip from London, just to see and walk a bit? We will be there in mid March. Is that going to be horrible in weather or possibly ok? Is that a train ticket I book in advance or just at the station? If at the station, is it a ticket booth /agent purchase, or online? Thanks
Technically, you can do Dover and back in a day by train. Buy ticket at station - agent or machine.
However, seeing the white cliffs is another matter. You could take a taxi to a viewpoint, but when you are standing on the cliffs you don't really see them.
I don't think there's a bus service or tour, and it's definitely not walkable from the station.
The views are from the sea, and for that you would need a cross channel ferry - making the trip much more complicated.
There are tours from London which offer a stop for a glimpse, as well as other sights, eg
I just googled day trip to white cliffs.
Weather could be anything.
We have done this and it's relatively easy and straightforward. As mentioned above, you can't technically see the cliffs when you're walking on them, but it's a lovely walk and on a clear day you can see France pretty easily. March weather is a bit unpredictable so it's hard to say. If you book the train in advance it will be less expensive generally. Since the fast train to Dover is on the HS1 line it tends to be quite expensive so is worth booking ahead. You can do it from nationalrail.co.uk. The fast trains go from St Pancras and the slow trains go from Charing Cross. If you have time I would also visit Dover Castle.
You can find more info here: https://www.nationaltrust.org.uk/visit/kent/the-white-cliffs-of-dover
You take the new hourly bus #66 at 20 minutes past each hour from Dover station to the Docks (technically the Eastern Docks), walk straight back out the side pedestrian gate of the Docks, and you are on the Dame Vera Lynn Way. That goes along the base of the cliffs, so close that you can touch them.
The Dame Vera Lynn way actually starts from the old Western Docks (now the Cruise Terminals).