SDLT exchange or part gift

Not my subject but I have an SDLT query
Clients, who are parents and son, had originally intended to exchange properties with difference in value gifted from parents. However the SDLT position on an exchange Is different from the alternative of a gift of one property only, with part purchase by son despite HMRC not normally charging SDLT on any gifted element.
This is because the gift element is not ‘chargeable consideration‘ for SDLT purposes, although, under a deed of exchange, the chargeable consideration is defined (FA 2003 Schedule 4 para 5 and sub para 3(A) as the market value of the acquisition. Thus, if one house was say £2mill and other was £1.2 mill, the gift element would be £800,000 but, if exchanged, then, despite this, SDLT would be payable on £2mill by son, despite only £1.2mill consideration and on £1.2mill by parents i.e. £2mill deemed to have been ‘paid’, less gifted element of £800,000 Thus, on an exchange, the gifted element of £800,000 mill is taxed in relation to the son’s acquisition
If, instead, parents merely transferred their property to son for £1.2mill, the purchase price element of £1.2 mill would be the chargeable consideration and the other £800,000 would be the gift, so not liable to SDLT or have I missed something?
Michael Jepson
M J Consultants