I'm not sure how flightradar24 functions and I'm just watching it for the first time. It's currently showing two aircraft in the London TMA whereas my SBS is showing 50+ in the TMA out of 113 total over the UK. I guess flightradar24 has to be set up somehow, but I'm not aware of how to do that.
I did some lengthy investigation of the various ADS-B receivers out there on the market and how they can be used. Since I haven't actually done this, don't count what I say as authoritative but I believe I'm approximately right.
Sites like flightradar24 and radarvirtuel rely on volunteers with ADS-B receivers to cooperate via permanently-on connections to the internet. Those sites aggregate ADS-B information from the volunteers and then present it graphically. They will have different sets of volunteers in different locations with different reception coverage, which accounts for different aircraft showing. Also not all volunteers leave their receivers online all the time.
What you see is what information they have aggregated at that moment, not necessarily a complete representation of the aircraft actually out there.