YANBU @babypickles This is deeply odd. Who does that FFS?! 
Not gonna lie, I'd be telling them to stop, and if they don't, I will call the police (101.) I'm not joking. I would if they didn't cease and desist.
Ignore people saying they're not doing anything wrong. They ARE if it's bothering/offending neighbours.
In England and Wales, it is not an offence to be naked in public, but there are caveats attached as you cannot be nude with the intention of causing "distress or alarm" - as this is an offence under Section 66 of the Sexual Offences Act 2003.
The Crown Prosecution Service says: "In the case of naturism a balance needs to be struck between the naturist's right to freedom of expression and the right of the wider public to be protected from harassment, alarm and distress."
While you might be on your own property while in your garden, most people living in terraced houses and semi detached (and even detached in some cases,) will know that it's often fairly easy to catch a glimpse of your neighbour's garden, even if you're not intending on snooping.
This means that even though you're not necessarily naked in public, you could still cause "distress or alarm" to your neighbours - although you're only breaking the law if you "intended" to do so.
So yeah, tell them to stop, or put screens up if they are so desperate to be bloody naked.