I have some experience of this. You may well be right about mortgage lenders not being materially affected by a stamp duty scheme. However, most solicitors will insist on telling them about it and many lenders are not happy to lend where stamp duty schemes are being used. Especially state owned...
There are a number of stamp duty avoidance schemes which are perfectly legal. Usually only worth doing if you are buying for over 500k or £1m - otherwise the costs are a bit prohibitive. Have to wait for nine months - after which the IR cant come after you. The IR are always looking for ways...