Funnily enough, we were at the PI embassy in London the other week and spoke with the consul there. As always, it's impossible to get a straight answer and I'm still not quite sure of the process, and to be honest, i don't think she was either, but perish the thought she would "lose face" by being honest about it.
From what i could gather, even though british citizenship has been obtained, if you have a PI passport and it's due to expire and you want to renew it, then you can renew it - as they haven't a clue whether you've obtained british citizenship or not (similar to what happened to Pauldo, I think...?) However, if you don't bother to renew the PI passport and then try to obtain one later on (having let the old one expire) then you MAY need the oath nonsense. Or you may not. Like I said, I don't think she knew herself, and was obviously uncomfortable at the loss of face she was already experiencing at our suggestion we'll probably just let the PI citizenship slide because, let's face it, who would want it when you have a british passport?
So, I think, if you really do want to retain dual citizenship, the easiest way to do it is to renew your PI passport upon expiry of the old one, and ignore their oath taking suggestions. Don't forget, it's actually harder to open a bank account in the high street than it is to get a philippine passport so, like Pauldo, providing you're simply renewing, you shouldn't have too much trouble, british citizenship or not.
