In short she needs to be free to marry. For the optimum all round outcome that means Philippine annulment.

It's quite an issue really.

What's her immigration status?

She could instigate divorce here in UK based on 5 years apart, (living apart for more than 5 years is usually enough to get a divorce, even if your husband or wife disagrees with the divorce.) but since she is a Philippine national and also the fact that she would be the divorce petitioner it could never be a legal / recognised divorce in Philippines.
(unless she can secure British Citizenship first)
Legally/Technically if they were to set foot in the Philippines it would be a case of bigamy at best. That's a serious criminal offence.