We're talking about Inheritance tax here, not CGT?
There's no such thing as "losses incurred previously" for the purposes of Inheritance Tax. That would imply that you had somehow taken an inheritance with a negative value at some point in the past.
I presume the OP is talking about Capital Losses, on assets sold at a loss previously (eg. bank shares)... these have nothing to do with Inheritance tax, and any such loss can be carried forward against future Capital Gains (eg. upon the sale of an ihnerited asset for greater than it's value at date of inheritance).