If she applies for a visit visa she MUST mention she is in a relationship with you, not doing so has caused complications in the past with other visa's.
Proof of relationship usually means you have to have been the Philippines to show commitment, not in all case, but the MAJORITY... yes, and it definitely helps with spouse visa's.
It's best to keep it as simple as possible and truthful from the outset with the UKBA.
Annulment prevents problems in the Phil in the future, Westerners have been arrested and jailed for marrying a Phil that has not been annulled. It can be done within a few months, ask in a new thread.