IME, whether you buy online or via a manually completed proposal form, one of the questions asked usually is "Are you the registered owner of the vehicle?".
If you answered "No" to that question, you'd have been asked more questions, like "Who is the registered owner of the vehicle?" and maybe even "Does the registered owner of the vehicle already have insurance?".
So I don't know how you are fixed in the circumstances you outline. If you have valid comprehensive insurance, doubtful I think as it sounds like you have no insurable interest in the vehicle, you may be able to claim on your own policy.