First of all Class A airspace is permanant IFR. If you don't have an instrument rating you cannot transit class A.
Not strictly true since helicopters can use the heliroutes in the Heathrow CTA (class A) and I believe there is a corner cut that FW can use (Burnham?). Denham and Fairoaks also lie within the Heathrow CTA but have notified routes. Hang on a minute I've got a Pedantic caption!
SVFR is very confusing but as has already been highlighted it is a mechanism by which VFR traffic can access aerodromes which class D would normally prohibit. I believe it also absolves one from the 1500' (or is it 1000' now) rule, but not the land clear in the event of engine failure.