TextPlus App available on UK iphone?

I just moved from the US to England. In order for my friends and family to not pay international texting charges, I want to download a free text app on a UK iphone. While in the states, I had them all, TextNow, TextPlus etc.... From what I can see on the UK pages and after visiting one UK shop, it looks like the only app available is called TextPlus. I'm not 100% sure if TextPlus was the one I saw but I think so. The UK do not have any text apps that allow free texting within the UK. I wanted to know if the TextPlus app was available on the UK iphone, and if so, I presume it will give me a US phone number? The guy in the shop did not know much and said that the US iphone apps were different to the UK iphone apps. My friends and family in the states are all on different carriers, different phones, (iphones, blackberry, android, and basic cell phones), so I really need a US number so I can text their US number regardless of what phone they have. If I'm wrong with the name of the app, does anyone know the right name? The man in the shop did a search for me and came back with just one app. The info said it would only work with a ten digit cell phone number so I know it's US as UK cell phones have more than ten digits. He couldn't install it for me in the store. Thanks so much.