In a low pressure situation it is not unusual to see minus mode C values...especially over the sea and low lying terrain. To correct an earlier contribution, submarines are not equipped with mode C transponders.....naval vessels are a regular feature around the UK coast and are frequently observed...particularly in The Channel. Sadly my ATC unit can find no dispensation in MATS 1 or MATS 2 or from SRG that would allow us to ignore warships.....therefore when providing deconfliction service we have to avoid.
n.b. codes are published in the assignment plan.