Im sure you have searched because there have been loads of threads on this subject as its an importnat one.
Have used xoom which is pretty good but takes time and your money unless im doing it wrong gets changed to dollars and then pesos so exchange rates can affect you if that combination of exchanges is not favourably at the mo.
We also use phill remitance where we pay the money into there account and then let them know where we want it paid to and its send normally a better rate. Looking at PNB where you can open an account and simply do a transfer people say its an even better rate. There are a few well established companies in the UK who have more convient and cheaper services for particular banks both here in the UK.
With regards western union which is one of the best in an emgency ie they have agents all over the place in the UK and Phill and the money can be there in mintues. The missus got sent a card which gives her a discount of about 50 percent i think on the fee (will have to check that). Also if you can wait two days you can get a half price rate with western union as well which with the card makes it not to expensive.
Also a quick note be careful if using a credit card with any of these companies as it may be treated as a cash withdrawal which can cost you more in intrest and possibly a fee i guess.
There are quite a few remittance companies which are long time established in the UK, but do be careful as in london there was recently a collaspe of a company which speclizised in remittance to bangladesh and the report highlighted that there are few regulations apart from money laundering laws to look after the consumer. But now a few mps have realised its an important thing to a growing number of people around the country so hopefully something will be done.
