day trip from london to portsmouth and winchester

You should move this question to the England forum - you will get answers more quickly there. Sorry I can't help with your specific question.

That's a very long day - 2:17 on Eurostar to London from Paris after 00:45 to 1 hour check in time (if not checked in , not waiting in line to check in, both in London and Paris, an absolute minimum 30 minutes prior to booked departure you don't go) then having to transfer across London from St Pancras International station to Waterloo to get the Southwest Trains train to Winchester or Portsmouth, quite a long journey; then you have to do it all again going back - some day.

Nigel.. I think although he incorrectly posted this in France forum,, he meant as a daytrip from London?? I was curious also.. as a daytrip from London is Portsmouth very far?

Depends on the service,number of stops, etc., but from 1:40 to 2:15 or so, each way Waterloo to Portsmouth Harbour or Portsmouth and Southsea.

An hour or so each way to Winchester.

hmmm,, that makes it about a 4 hour ( or more) round trip.. doable I suppose , but desirable, maybe not. I visited it once years ago.. but was with local friends who lived closer to it ..