When we discussed this question in May, here, I suggested that Rail Travel Vouchers (RTVs) are Transferable, but others quite correctly pointed to the wording "Not Transferrable" on the front of many of these RTVs.
Where that restriction is printed on them, then it's clear that the restriction applies.
But you can buy a ticket for someone else using RTVs. They're just not transferable once they've been bought for a particular person