I seem to recall asking similar questions back in March before the change came into effect. I eventually came across this thread on pistonheads:
www.pistonheads.com/gassing/topic.asp?t=1193332There is a reference in it to a letter from VOSA stating:
"Thank you for your email enquiry dated 22nd August 2012, concerning an
airbag.
Until recently, the content of the MOT test was the domain of the
Department for Transport and is enshrined in legislation. The test also
complied with the European Framework Directive on 'Periodic Technical
Inspection' as the MOT test is known in Europe.
However, this Framework Directive was recast in 2009 (2009/40/EC) and
amended in 2010 (2010/48/EU) to include numerous new mandatory test items.
These changes are aimed at bringing the MOT test more up to date by
including the inspection of electronic systems which are now a fundamental
part of vehicle design. They are also a step towards harmonising the test
standards across the EU.
As an EU Member State, we are legally obliged to comply with this Directive
and to introduce the new test items by 2012. The reason that these new
items have since been 'deferred' is because there have been delays in
amending our domestic legislation to accommodate the changes.
Failure to comply with the Directive is not an option and would lead to
infraction proceedings being taken against the government, resulting in
fines, potentially running to millions of Euros and continuing until such
time as the Directive is complied with.
Whilst the Department for Transport would have commented on this Directive
on behalf of the government at the proposal stage, and may well have
opposed some aspects, we now have no option but to comply with the
requirements of the Directive.
In respect of the SRS (Supplementary Restraint System) warning lamp; a
vehicle will fail if this lamp indicates an SRS system malfunction.
However, there will be no MOT requirement for the lamp to actually work
once these changes become 'live'.
I hope this information has assisted you with your enquiry, but if you have
any further questions please do not hesitate to contact us again."
I don't know the person who posted it so cannot verify the provenance of what VOSA apparently said. If I can be arsed this weekend, I might email them myself.