It's all on the Ofcom site if people care to read it:

Can community stations broadcast adverts?

Each station can carry advertising and sponsorship, although there are rules on how much income they can take from these sources (income above £15,000 from advertising and sponsorship must be balanced with additional income from other sources). A small number of community stations – where they overlap with small commercial services whose studios are not co-located with other stations – are restricted to a maximum of £15,000 from advertising and sponsorship.

What does not for profit mean?

It means any profits generated by the community radio station cannot be given to shareholders for example, or to benefit the people running the service. However, this requirement does not prevent stations from paying staff. Any profit or surplus must be used for securing or improving the future provision of the radio service or for delivering social gain/community benefits to the station's target community.