SVFR is allowed in a control Zone / Class A. The limiting factor for SVFR in control zone is the fact a clearance won't be issued with vis <1800m or ceiling <600ft
Other than that its - clear of cloud and in sight of surface plus
"The pilot must at all times remain in flight conditions which enable him to determine his flight path and keep clear of obstacles; "
http://www.caa.co.uk/docs/64/VFR_Guide_2011.pdf p20 SVFR