I'm assuming that you will own the company. If that is the case, then the sales to the company will be deemed to have taken place at the open market value. There is an article on the topic here
, though the way SDLT rates are applied has changed since it was written. The HMRC notes are here
. See SDLTM30100 and the section on companies.
There was a case a year or so ago where a taxpayer successfully argued that her letting activities amounted to a business for the purposes of incorporation relief. You can read about it here
. This is not rollover relief which is where one business asset is sold and another one purchased. This is incorporation relief which is given automatically to unincorporated traders who incorporate and allows gains to be deferred. Whether HMRC will appeal further I cannot tell you.
This case will give false hope to many buy to lettors. Most in my experience take no part in the management of their properties which they have an agent do. Successive governments have never seen property letting as a proper business, rather an investment, because it doesn't really create anything though letting agents may argue otherwise. It certainly has led to price inflation that might not have happened without a buy to let boom. Attitudes do change and it may be that letting will be given access to tax reliefs that other businesses get now in the future but most of those who try to use the Ramsey case will fail I'm afraid.
I hope this helps but let me know if you have any further questions.