I also fly in southern England and think my SE2 is positioned in a very good location.
It is located above me and has a good unrestricted view forward and, probably, 70-80 deg either side of "straight ahead".
Typically, on a good flying day, I may only detect a few ADS-B Out aircraft and one or two FLARM targets if gliders are about.
You won't "see" all the commercial ADS-B traffic high above you.
If you also use your SE2 to transmit ADS-B Out you can check it out using the PAW vector application.
This gives a good indication (over a few flights) as to whether your SE2 is positioned in a good location and not masked by crew, engine or other metal bits of your 172.