BPR seems unlikely. Sounds as if the business is a property letting business [s105(3)]. Also see Pawson ; Zetland  and Best .
Whilst BPR refers to a "business", gift hold-over relief for CGT [s165] refer to "trade", so again hold-over seems unlikely to be available.
Presumably widow entitled to former spouse's unused RNRB. In which case for 2017/18 aggregate available RNRB amounts to £200,000 in which case no restriction if estate below £2.4m?