If you have an IR that is valid (in a legal sense) for the aircraft you are flying then you can fly in UK Class A airspace. Just tell them that's what you want to do.
Genghis is right though, IFR GA in the UK is relatively unusual - certainly on airways - and so it is normally assumed that a light aircraft won't be able to take an airways clearance.
One other point that shouldn't make a difference but is worth mentioning - London Information may not be manned by an air traffic controller. Like I say, it shouldn't make a difference but .....