Legally, Phil can leave his money to whoever he wants to.
However, anyone who is financially dependant on Phil can make a claim on the estate under the Inheritance Act 1975.
Let's assume Phil dies today with his last valid will being the one leaving everything to Lou.
Phil's estate goes to Probate so any creditors can claim for unpaid debts before the Residual Estate is given to Louise.
Unless a contract was signed by Sharon and Phil, Sharon is automatically entitled, as the surviving spouse, to one third of the Residual Estate. She just needs to apply to the Probate court and it will be granted. The only time Sharon wouldn't get anything else is if any joint accounts or properties that she held with Phil are equal to or more than a third of the residual estate.
Ben could contest the will if he could prove that he was financially dependent on Phil. So if the Will directed that all Phil's assets were sold/liquidated and the money given to Lou, Ben could be confident of getting it overturned as not only was he not left any money but the will also would make him unemployed.
Denny would also be able to claim based on his dependency on Phil (just - you need to have been dependant for 2 years) but the court could rule that Sharon's settlement would mean Denny was being cared for.
If Ben successfully challenged the latest will, Probate would then use any older will (and I am assuming there was one which benefited Ben, Sharon and Denny as well as Louise). If there is no Will, Phil would be intestate and that is a whole other kettle of fish.
If Phil dies without a valid will, any joint accounts or jointly owned property would become Sharon's sole property (not part of Phil's estate) and Sharon will automatically inherit all Phil's personal belongings, the first £250,000 plus half the remaining estate.
Phil's children would then share equally in what remained.