Texting from abroad in roaming mode is expensive, it cost you P20 to P30 depending on the network used.
Roaming may not be available with all GSM network (some are not recognized by Globe or Smart, it's not only in UK but in other countries as well). You can try to select manually your network and it may work. Usually with well known network (IE Vodaphone or T-mobile) it works.
If you do not want to spend too much with your load, use it only to receive messages and send them thru Chikka, Yahoo or Skype. That's the best deal.