Aren't these old chimney pots gorgeous; full of character? (Even with the more modern caps sticking through them.) I wonder if there is a reason for the crowned top or whether it was just designed to be decorative. I think these are queen crowns, rather than the bigger king crowns.
This moss covered stone is a telephone cable marker; probably pinpointing the position of a junction of cables. The letters G.P.O. indicate the General Post Office; the forerunner of British Telecom (back in the days before the mail and telephones were separated into different companies and the Post Office Tower in London became the Telecom Tower).
It's certainly looks to have been a while since this particular junction unwent repair work and I have no idea what the little envelope shaped slot is for; unless there was once a metal plate providing more information (depth of cable, number of cables in junction etc).