According to Wikipedia the dead were moved to a new cemetery, St Pancras & Islington, in East Finchley. I would imagine this was easier than moving them to another county as well as closer.

The City of London Cemetery seems to have been carved out of Epping Forest which is administered by the Corporation. There is a cluster of cemeteries there including Manor Park, Woodgrange Park, Plashet and not too far away East London (at Plaistow) and West Ham (at Forest Gate). It makes you wonder what would have happened if cremation hadn't become popular.