I would seek professional guidance on this but this might give you some background:-
"Keeping a former main residence
For someone who wishes to move house but decides not to sell their existing home at the same time, the higher rate of SDLT will apply. This is because even though they are replacing their main residence with a new one, at the end of the process they will own 2 properties. If the previous property is then sold within 36 months a refund of the higher SDLT rate can be applied for.
If someone decides to buy a new home as a main residence, but wants to rent out their existing home instead of selling it, they will be liable for the higher SDLT rate. This is because they will own 2 properties at the end of the process.
For a definition of a "main residence" for stamp duty purposes please refer to our stamp duty for second homes section.
I think these are areas where the detail needs to be clearer. If you are simply moving from house to house with a short delay in selling property 1, why should you need to jump through hoops to reclaim stamp duty on a new purchase?