i m a heating installer.
for my ten bobs worth i'd update the electric heating, you can do it one heater at a time to spread the cost.
putting in gas ch will be disruptive and as its a flat compromised by compromises.. ie a relatively new flat will have solid floors and ceilings.. where would the pipes go..
the cost would never be recupped and frankly you dont need it you ve probably already got an unvented hot water cylinder delivering lains pressure hot water you have to accept less than that with a combi..
no value in a conversion of your proerty is my opinion .. but its your money..