Most military aircraft flying around the UK do not transmit their long/lat position like commercial flights. Instead some (not all) just transmit limited information.
If this is information is received by 3+ ground stations (like as Raspberry Pi running dump1090) then the position of the aircraft can be calculated using a technique called multilateration (MLAT).
This is what 360Radar do, showing the results on a browser based map. Here’s an example of what you can see with a few Tornados (TOR), a Gazelle (GAZL) and a couple of Grobs flying around. It’s also common to see Eurofighters and Hawks showing up –