The best starting point for those researching English Presbyterian/Unitarian family history is Alan Ruston’s My Ancestors were English Presbyterians/Unitarians: How can I find out more about them? (The Society of Genealogists, London. First edition 1993, ISBN 0 946789 62 2, Second Edition 2001, ISBN 1 903462 21 5).
The Society maintains a location list of the records and archives of most Unitarian congregations in Great Britain.
It can be accessed in the Research section here : Location List
Details concerning individual congregations in the British Isles can be found on the following websites: