The civil partnership bill will probably address your concerns, but I don't know when we can expect it to be enacted but it wont be for a while, I think. I dont know when you plan to married, but you might be married before it happens. As you describe your partner as a fiancee, I assume you do have plans to marry soon.
First off, your fiancee will get nothing if you die tomorrow, unless you have willed it to her. If you have no will, your legal next of kin will get everything.
The exception to this is a jointly owned asset ( house, bank account).
Currently you and your fiancee are legally strangers,so there will be a tax bill on the inheritance.