Hi bigmac,
Your Fiancee was divorced in UK and that divorce is legally recognised virtually everywhere except in Philippines.
To be honest I've no experience or knowledge about the chances of your Fiancee getting her divorce legally recognised in the Philippines since she was the petitioner. Technically it's not possible.
Now whether or not she can succeed in securing an annulment comes down to finding the best combination of lawyer and judge.

Your challenge is finding the right route for your Fiancee to enter UK under a settlement visa.
Annulment would be one way.
Might be worth asking UKBA whether or not presentation of a UK issued divorce would be accepted for Fiancee Visa.......