-
Unless Armageddon pretty much any sensible* share tracker will out perform London rental property based on yearly yield alone factoring in fees and expenses.
Rental will get you less than 5% yearly, investing should over a good amount of time get you more when you average it back to yearly yield.
Over a long term timeframe it’s pretty likely you’d come out poorer if you keep the flat and rent it compared to if you did some fairly unexciting and hands off investing.
* moderate - low risk level, well diversified, global. Or just the S&P500
That's obviously based on no mortgage payment though (and some pretty low estimates like letting agency fees of 1%). You'd expect a decent return on that.
You could get a similar return, for a lot less work, with a S&P 500 tracker or similar so really it comes down to whether you want to keep the property to either live in at a future date or hope for significant increases in the property value.