Hi L'Argonauta,
It's just a prompt for you to conduct your cockpit checks. ATC will try not to talk to you or give any further instructions until you report "Checks complete". Additionally, a gear check is also a mandatory requirement prior to landing clearance for aircraft with retractable undercarriage. These 2 checks as well as a QFE/QNH readback prior to descending on the PAR glidepath are part of the UK military ATC procedures.
There is currently a move afoot to standardise mil and civ phraseology in the UK to
CAP413 standard, but don't hold your breath
Cheers,
LXGB