Have you looked at the British naturalisation documents in the British archives?
There are about half a dozen Demel's listed, documents should give all the info you require.
This one looks like yours and there is also a Hermann listed:-
Nationality and Naturalisation:
Demel, Carl Friederich Wilhelm, from Germany.
Resident in Manchester. Certificate 20790 issued 22 September 1911.
Rena in England