At this point, I do not understand how anyone can possibly believe that the people advocating for this stuff are thinking in terms of economic (as opposed to political or social) returns. This stuff makes no economic sense and is already bankrupting the country.
Also, there is a legal requirement for new builds to have this now, this is with a massive shortage of housing, with a government that is a government of the "people" but has just put out the same housing targets as the last one and is running 75% behind annually. The scale of subsidies being given to these industries is probably tens of billions, green energy subsidies are now 5x larger than industry profits...this makes no sense, even with sky-high electricity prices (to be clear, it is consumers who are ultimately paying for this...we pay higher prices so that a lawyer somewhere can prattle on about leading a "green revolution" that is lining the pockets of donors).
This ROI calculator looks reasonable: https://ukcalculator.com/solar-panel-roi-calculator.html - note that it subtracts the install cost for you, so any case where the final figure is positive is profitable. But of course that depends on whether grid prices go up or down in the next decade ...
I think a big part of the push to install heat pumps now is that it is understood that electricity production is in dire straights, taking into account that the transition to EV requires a lot of electricity.
For an 800W balcony system your background house usage is likely to be enough to self consume most of it.
You'd wouldn't be able to run even a small oil heater except maybe in peak summer.
It's a good match for working from home as it's a small amount of power spread over daylight hours.