What you need to do is have the employer write a letter saying that they are giving her a X amount of time off to visit you in the uk for holidays. also state on that letter that they expect her to come back to full fill the remainder of her contract.
TO be honest thats pretty much your best hope in getting this approved. (as others have said will be very difficult to prove that she will return if she doesnt have the job in Singapore.
Also if you are brave enough to book the flights before hand, make sure the return is to Singapore and not direct back to manila, this will help (mayeb not much but its something.)