I have found a way around it for you at www.ba.com!!
If you are actually flying BRU-LHR-BRU and LHR-MAN-LHR in the one trip, then you can book it as one itinerary (i.e. BRU-LHR-MAN-LHR-BRU) it will (hopefully) price as two separate returns giving you the cheapest fare but in the one booking. That way, BRU is your origin and therefore no problem with UK credit cards etc.
The way to do it is on the first page of www.ba.com, before the booking section there is "Book One Way and Multi City"......click on that. Then the next screen use BRU as your origin and MAN as your destination and LHR as your stopover....and go from there!
Hopefully that should work!!
If you need any more help let me know!