Well said Shadow.
The practice of 'control-authroities' (Police, Immigration and Customs) not being searched was been brought into question by a 2002 government report. The reports states that this leaves members of the organisations open to "intimidation, coercion or bribery". It says that discussions are on-going with all three authorities to seek a suitable alternative.
The report can be found online (in pdf format)
HERE - have a look at sections 9.7 - 9.9 on page 9.