The diffence in stamp duty suggests your daughter is looking at a £430k property and you are comparing first time buyer stamp duty (£6.5k) and second home stamp duty (£24.4k). Is this right?
Basically your daughter can no longer get first time buyer stamp duty. Even if she sells her existing property, she would have to pay standard stamp duty (£11.5k based on figures above). Her fiance could go it alone but obviously that rules out your daughter's income on the mortgage and she would have no interest in the property. An £18k saving though (assuming fiance is a FTB).
Transferring flat to Ltd Co (as above) is an option but be aware that Stamp Duty would be payable on market value of the flat and at the higher rate. Stamp duty for the new purchase would then be £11.5k (at standard rate) saving £12.9k on the second home rate.
Above is only from what I have seen from investigating a similar issue with a friend, it may or may not be accurate but might point you in the right direction. I would suggest speaking with an accountant to help you before making any decisions.