Improper use of 121.5 cannot be condoned. It does not belong to D&D, it is an international distress frequency.
A Training Fix is useful for both flight instructional and D&D training purposes - it is emphatically NOT a supplementary navigational technique for the inept.
If you are lost, admit it to yourself. Then, unless you're already working another unit, get onto 121.5 and admit it! It might cause some airliner-driver to have to turn down the second radio for a few minutes between waypoint watching, but better to disturb his hugely demanding day by calling 121.5 than to disturb it by blundering unannounced, uninvited and unwelcome into the approach path of the airport he's trying to land at.
D&D will be able to identify you and, if you need it, hand you off to another unit to give you any necessary navigational asistance.
Old-fashioned 'lost procedure' is increasingly becoming irrelevant in south central and south east UK due to the airspace complexity. So don't faff, call if you need to and don't pretend that you are only practising when in fact you need real assistance.
In the UK, GPS should be considered a supplementary VFR navigation aid unless the aircraft is fitted with an IFR-approved system. Even then it is an area navigation aid only and under no circumstances an approach aid. Yet......