"We are a small British company up against a monolithic, faceless Goliath."
Yes, and you always were and you knew it when you signed up.
It's only that now, you are feeling just how small you are and how Goliath Google is, and that feeling is not comfortable at all.
A wise man once said : do not put all your eggs in the same basket. You should have diversified, that would have lessened the blow.
In any case, you have little choice now but to educate your users in how to accept the security warning.