Pakistani authorities have named British Al-Qaeda suspect Rashid Rauf as a "key person" in the alleged plot.
Early editions of Britain's Sunday Observer newspaper quoted security sources as saying up to two dozen investigations were now underway across Britain.
The Sunday Times said that one of the suspects under arrest could be "Al-Qaeda's leader in Britain".
Stringent security checks are continuing to cause chaos at London's Heathrow airport, with long delays and flight cancellations. The airport cancelled one third of flights overnight in an attempt to return to a normal schedule.
British Airways has criticised the British Airports Authority for failing to have contingency measures to cope with the extra security.