Well, this incident happened in London, so legislation in the US won’t help Hugh Grant.
You are correct that it is “legal”. It is actually a special case the law recognizes for people that have a “public life”, where they are not entitled to the same level of privacy you or I could demand the police and courts enforce.
Obviously, I do not think it should be legal. Or, if they are going to strip celebrities of legal rights to privacy, the celebrity should be legally immune for actions to defend their privacy themselves — as Hugh Grant did, or by sicking their bodyguards on the paparazzi, etc.
To me, a stalker is anyone that tries to impose on your privacy when they are aware their attention is unwelcome.
Standing around outside someone’s home and waiting for some opportunity to surprise them while they are just going about their daily home life is perverted. This is not like taking pics of someone walking down a red carpet at the Oscars where they have had time to prepare their public persona — this is their life at home, where they should be entitled to have a bad day, crappy mood, toothache, sweaty, and dirty without seeing it splashed on some tabloid.
If the only thing this guy really did was ask Hugh Grant to smile for him I doubt very much this incident would have taken place. What these people want isn’t a smiling celebrity. They want them enraged, caught off guard, at their worst. That’s where they get the money. I just wonder what comment this Ian Whittaker actually said to Hugh Grant to set him off.