Visits Legislation.
Schools should refer in the first instance to Local Education Authority codes of practice for health and safety, and visits legislation. In addition the following website gives useful guidance on visits -
www.dfes.gov.uk/h_s_ev/It is good practice for teachers to make a preliminary visit prior to the children visiting the place of worship, not only to undertake a risk assessment, but also to identify the focus and activities for the visit. In addition, by meeting with the local religious leader, a teacher can ensure that s/he is made aware of the educational nature of the visit, the different backgrounds and ages of the children if appropriate, and also make sure s/he is happy to answer questions that the children might ask.
A sample permission letter (PDF, 46k) for a church visit is available which can be modified to suit your needs.