Do you have cash credit on your account to pay for UK/international texts which are not covered by membership plan bundled allowances?
Do you have an active membership plan?
Are you using the correct +44 ... prefixes for the number?
Are you sure that you're not trying to send a UK MMS rather than an SMS as the former are not supported?
What happens when you try to send a UK text message and it fails?
Any error messages?
If you need to contact 48 support about this then see the links in my signature below.
