How early should I buy train tickets for travel in England?

National Rail tickets tend to be released three months in advance. The general rule is that the further away you're going, the earlier you buy the better. For the journeys you've listed, I imagine the price won't increase significantly between release and the day of travel, but it will still be cheaper to buy in advance. I would advise buying 2-3 months in advance.

When you view prices at www.nationalrail.co.uk, they will be described as Advance, Off-Peak (after 9:30 a.m. weekdays), or Any Time. And if you are shown advance fares, you can click on "more fares" to compare the Any Time price. Buy your London-Windsor ticket as a roundtrip, not separate tickets at each end.

The ticket prices from Bath to Salisbury and Salisbury to London don't change prices. If you look at prices tomorrow and at the furthest day out you can currently purchase, they are always around £10.50 for the Bath to Salisbury leg and the Salisbury to London leg ranges from £38.30 to £39.70. If you are going from London to Bath, then on that line you can get a discount on an advanced purchase. Tomorrow the prices are showing as ranging from £32.50-43.40 while looking out to July they are available at £14.50. Huge difference there.

I researched this a lot before I went to London ->Salisbury and Salisbury ->Bath last year and just wound up going to the station both in London and Salisbury and buying a ticket that day. There were some minor variations based on times but not enough for me to be concerned about. It is a good idea to print out a list of train times so you don't have to wait long at the station.

Are you going to take the local bus out to Stonehenge? I did that last year and it worked great. Just buy your ticket from the bus driver...very easy.

Thanks to everyone for the information. It looks like I can just buy my tickets at the stations.