With the current situation in the Philippines ,..I am not sure how long till they give u a slot to submit the documents but yes book it as soon as all your documents are completed. ..I think you can pay it online but you also need to download the bank cert verification system form which you need to bring upon submission of the documents allowing the ukba staff to verify your bank accounts and it authenticity.
check the news and update for visa4uk :

Visa4UK unavailable 19 November 2013

http://www.ukba.homeoffice.gov.uk/si...ber/14-visa4uk