Thomas - thanks.
At a quick glance, from this AIC Mode S looks like this is being phased in from March 2005 to March 2007, with exemptions if, amongst other reasons, fewer than 30 hours annually are reckoned to be flown within designated controlled airspace, at present London TMA.
Assuming no airways en route and most controlled airspace flying under SVFR, 30 hours would cover a fair amount of IFR such flying, which are really only likely for approaches, I would have thought.
Clearly needs looking at in detail though to establish what real limitations there are going to be, and when.