It would only become an issue if they were to die within 7 years of making the gift.
So, no tax is payable at the date of the gift, but if they were to die within 7 years, then tax would become payable if their Estates were liable for Inheritance tax.
I hope this clarifies the position.
I do apologise- I have just re-read your original question.
As your parents woyuld be living at the property, the 7 year rule would not apply, and tax would be payable as and when they died, whether it be in 7 or 27 years time.
I do apologise for misleading you earlier.
I can, however, confirm, no tax is payable at the date of the gift- it only becomes payable when anything were to happen to your parents.
I would be grateful if you could leave positive feedback.
No income or capital gains tax would be payable on the Sale of your parents house-
Capital gains is only payable if it is not your principal place of residence.