I really don't think the HR department in your company knows how to deal with this. Assuming you've sent the Fit Note in to them, that is the Note that demonstrates that your doctor is saying you are OK to work. It is not uncommon for the company to want you to see a company-appointed doctor for a second opinion (at the companies expense), but it is usually the other way around - the employer wants the employee back but the employee won't come back to work.
Phone the employer, say you are ready and willing to come back to work, you've sent them the Fit Note, so can you start tomorrow, please. See what happens.