You need to clarify your post with regards to we, my, our and I etc
In March 2016 we purchased a BTL flat in my name only.
So is this flat owned by both you and your partner (albeit with you acting as a nominee) or is this solely your flat?
We are now looking at selling my partner's house
So are you and your partner selling your house or is your partner looking to sell his house?
getting on my partner's deeds
Merely as a nominee? Presumably not given your later comments.
My point to the solicitor was that the house has already been purchased paying the relevant SDLT in 2015
But the acquisition by your partner in 2015 is a different land transaction to the proposed acquisition by you now.
I am being give in my partner's house..?
But you're not being given it. You are assuming debt.