For IHT and CGT purposes, it could well be construed as a gift or a sale "at an undervalue".
The difficulty you have with the property transaction is that a letter on a conveyance is not enough, the transactionneeds to be done by deed to properly convey the legal title.
As far as common intention constructive trusts or beneficial interests are concerned, it is the common intention of the parties which is of relevance and not necessarily the legal title (for more info see Stack v Dowden HL either this year or last - sorry I don't have full ref) or the fact that anything is put in writing.
It may be worth being more specific on the situation you have in mind - some very knowledgable peeps here will help if they can.