No, because you're effectively "gifting" the remaining 170k value of the house without paying the associated inheritance tax etc. It's a tax dodge and quite rightly dealt with as such my HMRC.
This is utter twaddle, you can gift whatever you like, as much as you like to a dependant, as long as you live for seven years after the gift.
If you don't then inheritance tax comes into it, but otherwise it doesn't, and in the case of a house sale, if 170k was a gift, this is below the threshold so there is no tax to be paid upon it at all.
Stamp duty is another matter.
As is if the mother continues to live within the property, rent free, after selling/gifting the property to a child.
If she isn't living there, then no problem.