Bus or Train ?

Simple -- check both schedule and see which is more convenient. Since you are staying in the Victoria Station area you are very close to the National Bus terminal so equal option for train or bus.

Bath, Dover and Canterbury you can do easily by bus from Victoria. Salisbury has only 1 bus a day- evening from Victoria, morning from Salisbury. So that has to be train from Waterloo.

There are trans from Victoria to Canterbury (90 minutes) and Dover (2 hours) which you may find more convenient than tube to Kings Cross St Pancras then the High Speed service- 1 hour and 1 hour 10 minutes respectively.

Bath by train is from Paddington station.

It really depends on what is important to you....cost? times? comfort?

Since these are day trips, take into account at what times the services operate and how long it takes to get to and from your destination.

And keep in mind, not all your train trips will depart from Victoria Station. Although you can get the tube to each of them from the Victoria Underground station.

Thanks to you all for your insightful advice