Hi.
Provided they haven't lived in the house with you in the last 3 years, and provided you haven't got a beneficial interest in any other houses, then it looks like you qualify for the exemption.
Even if you weren't entitled to the relief, any moneys you've paid towards the mortgage could be considered consideration (i.e. you continued paying the mortgage on the basis that the house will become yours), and the value of the gift would be reduced accordingly.
As regards CGT, your parents will only be liable for CGT for the period that they have occupied the house as their principal private residence - so say if they've owned it for 30 years, and moved away 6 years ago, then they are only liable for CGT on 6/30 of any gain. Also, this is probably a good time to be doing the transfer, as the value for CGT purposes will be the deemed market value of the house, which could be quite low in the current market...