chrisN,
The TMZs near Stansted justified the expression rather more, as I understand it, because the only exceptions are for people from local aerodromes with specifically defined procedures.
You understand incorrectly.
As usual the AIP has the requisite info...
Transponder Mandatory Zone Access
a. Suitably equipped aircraft may access a Stansted TMZ without ATC approval although such traffic is strongly recommended to afford itself of
either an ATSOCA service or make use of an appropriate Monitoring Code (see ENR 1-6-2-2, para 2.5) if available.
b. The pilot of an aircraft that wishes to operate in a Stansted TMZ without serviceable transponder equipment as defined in GEN 1-5-14 may be granted access to the TMZ subject to specific ATC approval. This approval may be obtained from Farnborough Radar on frequency 132.800 during their hours of operation (0800-2000 Winter (Summer 1hr earlier)) or from Essex Radar on frequency 120.625, at other times. S.I. 2009/2020 refers.