Unless the SMTP server for the account is authenticated, you will have problems sending with the account.
Is the SMTP server for this account authenticated?
If not, in the U.S. AT&T's SMTP server to be used when connected to AT&T's cellular network is included as an SMTP server to choose from and use with an email account. The same may be included for your carrier in the UK.
If so, this should work for sending messages with the account when connected to the cellular network, but may not work when connected to an available wi-fi network.
I access two email accounts with the iPhone's Mail client and both email accounts have an authenticated SMTP server. I've never had a problem sending messages with either account regardless the internet service provider associated with my internet connection at the time.
This restriction imposed by internet service providers is part of an overall effort to prevent spam emanating from the ISP's domain. You can thank low-life spammers for this. The same would apply when accessing an email account that does not have an authenticated server with an email client on a laptop.