Yes you can, and they can film you. There's no law against photographing - and by extension filming, anyone in a public place, even if they don't want to be filmed.
Using a long lens to film someone in their own home or garden, where they can reasonably expect privacy is different, but if someone's out in public, they can be filmed. Even children. Though in the current climate you'd be a bit daft to go out filming kids, there's no law against it.
Hi Eifon,
I think you are 101% correct in saying this about there being no law.I think it is just the PC do-gooders who like and who'd like to make people believe and think this more than anything.