To pay to transfer miles and to make cash/miles booking, makes no sense. In fact, it hardly never makes sense to transfer miles at a cost of 20 USD (or is it 30 USD?) per 1000 miles when you are able to purchase a TK award for a third party by calling TK CS (you can't do it online)

To spend 56,400 miles to get a reduction of 350 £ is poor value IMO (unless you have too many miles and nothing to use them for).