BA has a widebody maint base at Cardiff. All B744 and B777 major work is performed here.
They have a narrow body maint base at Glasgow. All B737 and A320 major work is performed there.
Most of the rest is actioned in London.
At LHR there is a B767 and B757 major line, and also hangar maint on all the LHR based fleet is carried out at LHR up to C checks, which are sent to GLA and CWL.
At LGW there is a hangar for minor maint. Also the B767 cabin conversions were carried out at LGW recently, but I don't know what that hangar is doing now.
BA does send some work outside these bases. Aircraft often visit Marshalls in Cambridge, and odd major checks are done by contractors. A few years ago the whole B742 fleet visited Avalon in Australia for major checks!
BMI has a maint base at LHR, and one at EMA.