Traveling from Waterloo Station to Windsor and Salisbury

Trains don’t sell out, with the exception of Eurostar. You won’t save money buying in advance.

Well, you can look for Advance tickets, which come out about 3+ weeks ahead of time, and those will definitely save you money. Figure out which railway you need for the various destinations, then sign up to receive notifications by email, and you should receive word when the Advance tickets go on sale (or news of other sales). I saved a lot of money doing that.

There are advance fares on the Salisbury route, but not on the Windsor route.

Whether you want to be tied down to specific trains on Advance tickets or prefer flexibility at a higher cost is your choice.

With 6 of you it is worth looking into whether you can benefit from the groupsave offer to Salisbury- https://www.southwesternrailway.com/train-tickets/ticket-types/groupsave-train-tickets (not available on advance fares).

Great Western Railway (for Windsor) also have groupsave- https://www.gwr.com/your-tickets/ways-to-save/cheap-tickets/groupsave

That means that the usual advice just to use credit cards to tap in and out to Windsor doesn't work for such a group size - where it will be cheaper to buy actual tickets- whether beforehand or on the day is your choice.

Paying by contactless to Windsor is £14.20 each off peak, on groupsave for 6 it is £59.60 for the group

If there is at least one child it would be worth looking into whether a £30 family and friends railcard will give you a cheaper overall fare than groupsave.

Groupsave and Railcard discounts can't be combined!!