Details in the UK AIP, specifically 1.5.3, para 1.2.2.
http://www.nats-uk.ead-it.com/aip/cu...GEN_1_5_en.pdf
The document talks about "IFR flight in controlled airspace". I don't see a distinction between A on one side and D-E on the other side, so my guess is that it would be required in B-E too. But I'm not IFR rated and don't fly in the UK all that often, so I might have missed something.