Can staff enter competitions?

We are planning a Prize Draw, inviting supporters to make a pledge for £10 per month for 2 years. In return they are entered into a Draw for a donated prize i.e. A week in New York! Does anyone know whether there are any legal implications about members of staff entering? I know that with businesses often employees cannot take advantage of competitions and offers but I wasn't sure whether it also applied to charities. Any thoughts would be most welcome!