Like many areas of the civil regulatory process, the mandatory requirement for TCAS doesn't apply to State aircraft. The following quotes are lifted from the Eurocontrol website with some of the jargon removed:
http://www.eurocontrol.int/mil/publi...e/cnsacas.html
ACAS Policy
The ECAC Member States have commonly agreed on an ACAS policy and a mandatory ACAS II implementation schedule. This mandatory implementation does not apply to State Aircraft.
However, the Military Authorities of the ECAC Member States agreed on a voluntary installation programme on military transport-type aircraft (MTTA) equivalent to their Phase 1 civilian counterparts by 1 January 2005.
Situation in German airspace
Notwithstanding that the military commitment is voluntary, Germany has made ACAS II mandatory within its airspace...from 1 January 2005...for all aircraft whether civil or MTTA..
The German withdrawal of State exemption was what finally forced the MoD into fitting TCAS into the TriStar and VC10.
The Nimrod is not regarded as an MTTA so it continues without TCAS.
The FAA regulations are, I believe, broadly similar.
The below-F290 issue is to do with RVSM regulations requiring carriage of TCAS. Again this does not apply to State aircraft, which can be RVSM certified without TCAS. In fact, State aircraft do not need an RVSM certification to fly in such airspace, but as the controller would then have to apply increased separation against other traffic it is not likely that ATC would grant a clearance for flight at those levels.