Don't know whether they're already in use somewhere, but it would seem to me that a VHF frequency close to 121.5 could be allocated for practice emergencies eg 121.475 or 121.525. It would require international agreement of course, as the frequency could potentially be used anywhere within UK airspace, but it would seem sensible to put an upper limit on its use eg FL100 in order to avoid interference with other users in neighbouring FIR's.