Your parents gave you a gift of a house. Now you want to give them a gift of the house. Assuming they dont live there, there is a possible charge to CGT depending on the values etc. Your parents can take up to 46673 each from you free of gift tax. After that, tax is at 20%.
The only way to avoid dealing with the above and indeed stamp duty is to enter in to a deed of trust saying that your parents put the property in your name but you held as trustee for them, and they were always the beneficial owners. But it doesnt sound like thats true.