For those will a faith it should be relatively straight forward to contact the appropriate person within their community.
For those wishing to have non-religious or secular ceremony you may wish to use a celebrant, here are some leads:
The British Humanist Association
BHA, 1 Gower Street, London, WC1E 6HD
Tel: 020 7079 3580
You can also find a selection of celebrants at www.independentcelebrants.com