Can't tell you anything about the particular flight 'cos I'm nothing to do with LHR but cat A is an internal UK ATC prioritisation assigned to "Aircraft in emergency (e.g. engine fault, fuel shortage, seriously ill passenger). Aircraft which have declared a 'Police Emergency'. Ambulance/Medical aircraft when the safety of life is involved".
It's not something that pilots have any need to be aware of (although, as you noticed, such a flight may have an impact on other operations) and I don't think it is taught during training routinely so a controller mentioning the categorisation of the flight on the RTF is usually rather confusing for aircraft on the frequency.
You can find more details in the UK controllers' manual
CAP 493 Manual of Air Traffic Services. As far as I am aware it's not in ICAO so you will probably find that it is not a categorisation that is commonly recognised outside the UK.