Hi,
I did think it was 21 days but recently saw this -

"British and Irish passport holders may enter and stay in the Philippines for 30 days without visa provided that they hold valid tickets for their return journey to port of origin or next port of destination, and that their passports are valid for at least 6 months beyond the intended period of stay in the Philippines."

http://philembassy-uk.org/consular-matters/visa