This guide is not by a lawyer, but it seems to have a good general overview of the law regarding photography (warning: pdf link
). Basically there are four aspects to photography as far as the law is concerned. This is US law.
1) Whether you have a right to take a photograph (e.g. there are laws that restrict you from taking pictures of some military bases whether you can see them from a public place or not)
2) Whether you have a right to be in the place where you're taking the photograph (e.g. with Stonehenge I presume it's private property so you if you run on without paying their admission fee you'd be trespassing)
3) Whether you have a right to publish the photograph (e.g. you can't legally publish an image of a copyrighted work as your MoMA visit indicates though surely fair use would apply to parody or the like)
4) Whether you have a right to make money off the publication of a photograph (e.g. you can't sell a photo you took of Brett Farve without his permission, but there has to be more to it than that because the paparazzi are always selling celebrity photos)
In general, those four rights are unrelated. That is, you can be trespassing, but still be able to take a photograph and publish it, you just might be arrested for trespassing. As for British law, I have no idea what your rights would be. The government has video cameras watching you on the streets in London, but I wouldn't be the least bit surprised if I learned it was illegal for a the general public to take pictures of those same cameras.