Traveling internationally, iPhone is sending iMessages but not text messages?

I'm from America but I'm living in the UK for a while. My iPhone will send iMessages with or without Wi-Fi, but will never send regular texts to android phones. This was happening when I had my American SIM card as well as my new UK SIM card. Any suggestions?

You need a plan that allows it, check with your carrier. Imessages don't care where you are and are always free.