Given that your primary residence has a mortgage, you won't just be able to transfer it to your parents - the mortgage company would normally first want the mortgage to be repaid. Even if they allowed it to be transferred, this would be likely to involve fees, including verifying that your parents and then you were eligible for the mortgage.
Even if your property were mortgage free, you would risk having the transactions be treated as an artificial way to avoid tax, and ignored. In the worst case you could be prosecuted for fraud or tax evasion.
If your scheme did work and your parents died within 7 years, you might find yourself liable to inheritance tax on the "gift" returning your property.