Ofcom broadcast code 1.16
The transition to more adult material must not be unduly abrupt at the watershed (in the case of television) or after the time when children are particularly likely to be listening (in the case of radio). For television, the strongest material should appear later in the schedule." They say for children that particularly applies when they are travelling to or coming home from school.
https://www.ofcom.org.uk/tv-radio-and-on-demand/broadcast-standards/section-one-protecting-under-eighteens

Ofcom Code 2.3
In applying generally accepted standards broadcasters must ensure that material which may cause offence is justified by the context (see meaning of “context” below). Such material may include, but is not limited to, offensive language, violence, sex, sexual violence, humiliation, distress, violation of human dignity, discriminatory treatment or language (for example on the grounds of age, disability, gender reassignment, pregnancy and maternity, race, religion or belief, sex and sexual orientation, and marriage and civil partnership), and treatment of people who appear to be put at risk of significant harm as a result of their taking part in a programme. Appropriate information should also be broadcast where it would assist in avoiding or minimising offence.

There's a guide based on public research conducted by MORI on offensive language broadcasters have a copy of. The Ofcom broadcast code applies to all television or radio stations that have a licence from them.

As Dave says this isn't about an offensive word on a track Caroline played.

Caroline Community Radio was found in breach of the broadcast code for playing a track with prolonged sounds of sexual moaning at 8.21am during the school run.
https://news.sky.com/story/caroline-community-radio-told-off-by-ofcom-for-playing-sexual-moaning-song-at-8-20am-during-school-run-12280376

I don't see any issue with broadcast radio stations having to follow rules about harmful and offensive material.