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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
In my case there is no mortgage so the decision is: keep flat and rent it out or sell flat to buy more expensive new place (or reduce mortgage or whatever having more money lets you do).
Where do you get your returns values for trackers like S&P?