Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
On death all a deceased's assets are aggregated and subjected to Inheritance Tax (IHT). IHT kicks in at 325K and is at 40% flat rate on the surplus. If over 10% of the estate is bequeahted for charitable purposes then the rate falls to 36%. The 325K is inflated by any charitable and inter spousal bequests also.
Inter spousal bequests are thus outside the scope of IHT. A anything received from your late wife esapes any UK taxation irrespective of the amount so the lump sum is yours tax free anyway.
I do hope that I have been able to set your mind at rest on this matter.