Regardless of any billing add-on, your ability to get coverage abroad (which calls, texts & mobile data rely on) is dependent on roaming being activated - which allows your phone to connect to foreign networks. Thus the reason you can't text is not because of no data (text messages don't use data), it's because you don't have coverage.
Have you used your phone abroad previously, did you receive a welcome text message on first arrival abroad? These are good evidence of roaming being enabled if so. In these cases, manually cycling through all available local networks often resolves issues - toggling flight mode can have the same effect.
If not...On PAYG, roaming is auto-enabled once your phone is used in the UK first. On pay-monthly, this needs to be enabled manually - usually by texting ROAMING to 150 before leaving the UK. Did you do this?
If you are on pay-monthly and now find yourself abroad without having enabled roaming, your only recourse will be to contact CS to request it activated. You can either contact +44 7953 966150 from another working phone, or +44 800 9566000 using a web-calling app such as Skype over WiFi.
