I'm thinking about building an online platform that helps people get second medical opinions from doctors in the UK. Is there a possibility that the doctors on the platform could get sued for medical malpractice? I don't mind putting a huge disclaimer that limits the liability if it works.
Yes. Anyone with the filing fee and a laptop and a printer can bring a lawsuit. And, if a medical doctor provides advice that falls below the standard of care for a physician and causes harm, it is not just possible, but likely, that a medical malpractice lawsuit would be brought. Putting something on the Internet doesn't relieve someone of liability that they would have if they did the same thing in person, as a general rule.
It is possible that a disclaimer could reduce or eliminate liability in some cases, but that isn't true in all cases. There are some kinds of liability that cannot be disclaimed or waived. The most obvious exception, which shouldn't be a great concern, is that liability for intentional acts can't be waived. But, that isn't the only limitation.
From a practical perspective, the sensible thing to do would be to take whatever steps a doctor providing a second opinion would provide in an in person consult and include them in an online format.
It might be possible to entirely or significantly disclaim medical malpractice liability for the matchmaking aspect of the app itself, and limit liability for medical malpractice solely to the physician providing the advice and not to the app as a deemed employer of the physician, however.