Current law is a bit of a mess.
Essentially, the Rules of the Air regulations 2007 has been superceeded by Part-SERA (December 2014) but Part-SERA didn't cover everything so some parts have been retained, and there have been some derogations in some areas.
See http://www.caa.co.uk/docs/2884/20141204ROTA2007Retentions.pdf
and you can read about further details here: http://www.caa.co.uk/default.aspx?catid=2884&pagetype=90
This should all be cleaned up with new legislation this year.
However the booking out regulation hasn't been retained. It is no longer law for pilots. It is now only the Part-SERA regs which requires submission of some sort of flightplan (which could be an abbreviated flight plan, which the CAA helpfully refers to as "booking out") to be in controlled airspace (so likely including an ATZ).
Hope that clears the legals up!
Of course, might still be required by each organisation according to their own rules / safety management system.