I hardly ever get a mil ac NOT transmitting alt. I get AAC and RAF helis around here at 250' to 1500' always transmitting alt and I have NEVER had a US Mil ac not transmitting alt. That includes F-15s at LN.
Yes, I'd expect the Debden Mode S radar to see low-level traffic over much of East Anglia.
Bear in mind also that if an aircraft drops below or beyond radar coverage, the altitude shown on BaseStation/RadarBox doesn't disappear, it just stays at the last known value.