I personally would try two different options.
SignsofGod. They coordinate the Christian Interpreters Network. Many of Signs of God people are trained in church services and are able to sign many of the hymns etc that other interpreters find difficult. These interpreters are based across the country. Signs of God will put out an email for you and you can see if you get a response.
www.signsofgod.org.uk, gill@signsofgod.org.uk
The second place I would look is the ASLI directory. This is a list of interpreters many who work freelance.
www.asli.org.uk/interpreter-search-p3.aspx
Prices will vary between interpreters but you can ask for quotes.
If you get really stuck...you can contact me if you like - I'm on both the above lists and have interpreted other weddings but could be expensive because of travel costs??
Anna Smith