You'd just need a 52A junction box for the end of the original line cord to get that working. (Plus a parallel extension bell if desired.)
GPO/PO Telephone 776 (Compact) and Jubilee Compact
Telephone is wired to standard diagram (N876) but modified as follows:-
Telephone No. 776 and Jubilee without Bellset
Remove strap T2 - T3.
Insert strap T1 - T2.
Blue of line cord to T1.
Red of line cord to T4.
White of line cord to T5.
Green of line cord to T6.
http://www.britishtelephones.com/pstconv1.htm#776
http://www.samhallas.co.uk/repositor.../0000/N876.pdf