Freehold/Leasehold interest

A few years ago i converted a three bedroom house into two flats. All necessary planning/building regs were obtained but the deeds were not altered to reflect the conversion. The deeds show that the property remains as a three bed house, which is freehold. There is no mortgage attached to the property. We would like to consider borrowing more money on the property to purchase other rental properties, but lenders will not lend as the flats have no leasehold, apparently they need to hold their charge against the freehold and leasehold. We are happy to consider this but as we have no intention to sell the properties we do not know how to arrange this,presumably we need to sell the leasehold to an entity ( i assume the same person cannot own the freehold and leasehold).
This would then presumably have CGT implications (although i am not sure how you put a value on a freehold/leasehold interest).
My partner of 20 years is not on the deeds of the property so perhaps I could transfer either the leasehold interest or freehold (whichever is most tax efficient)?
In addition as we are currently paying 40% tax on our rental, we would like to consider arranging our affairs in a more tax efficient way (not avoid) and the changing of the deeds may be an opportunity to do this, however my concern is that any advice we get on the legal side would need to work on the tax side and i am not sure who we should seek advice from a lawyer or accountant ?

I have posted this question in the tax forum as the two issues appear to be linked.