Foreign property gift

My mother lived in UK and was a British citizen.   A few years ago she was concerned that she would have to live in a care home, and in that case her home would have been sold by the government to pay her care fees.  Her lawyer persuaded her to sign the deed over to my name to prevent this (making it a gift to me).   I am a US citizen, and reside in USA.  My mother had a legal agreement drawn up which stated that she had a lifetime right of abode at the address and that I could not sell it or charge her rent.   The lawyers also retained the title deeds until her death.  They registered the name change with the land office but the security was retained by them and not discharged until after her death (not sure what that actually means).   

 

I am not sure about the gains tax.  Do I need to use the date of ownershio change, or the date I was legally able to take physical possesion and my right to sell?   This makes quiet a difference in the gains tax. I did see some other articles which said this situation could be treated as an inheritance instead of a gift?