Philippines visa extension advice (British Passport)

What are you trying to achieve?

Are you talking about a Special non immigrant 47 A2 work visa?

You have two choices apply to the Philippine Embassy in London for a single or a multi entry visa and second turn up and apply for a visa extension once you are there.

It is possible to apply at the BI Office in the airport prior to the entry clearance desks but I don't know where that office is in NAIA Terminal 3 others might be able to help for other major airports, you can also just accept the 30 day on arrival clearance then apply for an extension at a BI Office nearest wherever you are staying.

Applying for a visa in London is a bit of a pain I did that back in 2005 and got a 59 day visa when I had a 36 day stay, I've also applied on arrival at the BI office in NAIA Terminal 1, you have to do that before clearing immigration, it's not hard and results in a fairly short delay in being cleared, all the paperwork is done in the airport.