Unless you have a big pile of USD in your top drawer, I cant see the point in taking them and getting ripped off exchanging GBP into USD and then changing the USD into PHP. You are paying commission twice and having the rate skimmed twice.
I've just come back from the Phil's about 3 weeks ago, using my debit card in the ATM's I was getting approx 73 PHP to a GBP, and at money changers in Manila was getting approx 72 PHP.
