LGW's fine if, as has been said, you arrive at a DOM airbridge - just hang around at the gate.
MAN, even though it's my home airport, I've never transferred so can't comment. Trust PennineUK.
LHR is the interesting one - I do DOM-DOM there quite a bit at the moment (all in the foolish pursuit of TPs and silver!), and a number of times (though fewer than half) the guy/guyette on the BP desk, bless them, hasn't got a clue what to do with a DOM-DOM transfer and has to "phone a friend", holding up your fellow travellers. Might be worth printing out the relevant page from the Heathrow website
http://www.heathrowairport.com/porta...0357e120a____/ to help them along ...