It's always difficult to diagnose email issues at a distance, but the most likely cause, I'd have thought, is that your service provider's SMTP server is rejecting the connection, possibly because it's coming from outside the UK.
If you use Gmail, try using the dedicated app as it requires no setup. If you don't, you should still have a Google account to use Google services with your (Android) phone. Try using their SMTP server instead. It should be SMTP.gmail.com and you'll need to set it to log in before sending ("my server requires authentication") using your Google username and password.
You could also try using mobile data, though this will, of course, incur a charge.
