As a matter of partnership law, the partners do not own a direct beneficial interest in (an undivided share of) the partnership assets. Instead they have a partnership interest which equates to an indirect interest the total net assets of the partnership itself, which is a chose in action, and so do not have a beneficial interest in them and therefore cannot (individually) transfer any such interest as a matter of property law, although the partnership can of course collectively transfer a partnership asset.
Although the analysis is not simple, I expect this can be done SDLT-free under Schedule 15 FA 2003 and if not perhaps you could first incorporate the partnership under s162 TCGA without SDLT and then transfer the shares.