S. 144 will not apply in the situation you describe. Carving out the BPR/APR assets is one way (probably the best way) to proceed. Alternatively it is common to have a two year discretionary trust (at the end of which the widow gets an IPDI in any remaining assets, which is read back under 144). Personally I sometimes don’t go quite that far, merely giving the trustees a power to accumulate income during the two year period (which has the same effect for IHT even if in practice the power isn’t used).
Either way, trust income will be subject to tax at the trust rate until the widow gets her interest in possession, and unless the re-arrangement of the estate can be done quickly post-death, there may be a necessity to pay IHT on the whole estate before obtaining the grant of probate, and then claim it back, which can cause cash-flow issues. It is good practice to include in the trust provisions a clause making it clear that the trustees can exercise their overriding powers before any grant of probate has been obtained.