Nominee is only trustee for legal heirs 

On husband demise you and your mother in law would be legal heirs having equal share in proceeds of insurance policy 

3) I presume you don’t have any kids 

Your mother in law ,you and your son will have equal share in insurance policy proceeds 

If the Insurance policy was purchased in India then it will be bound by Indian laws.

As per Indian laws, the insurance claim amount though settled in favor of nominee, the amount shall devolve on all class I legal heirs, therefore your mother in law is entitled to her rightful share in the insurance death claim amount.

If your husband purchased insurance policy at UK, then the UK laws will have an anser to your question.

If the insurance was taken in India, then the insurance death claim amount shall be disbursed equally among all the legal heirs of your deceased husband namely, you, your children and your mother in law.

If he was working in UK in a company falling under that country's law then if law permits you will be able to acquire the entire insurance claim amount and the terminal benefits.

So you may first clear the details and ascertain the facts in order to get proper opinion.

1. Nominee is only a custodian for company's insurance amount.

2.  The rights of legal heirs supercede the rights of the nominee. In other words, you, your son and your parents-in-law are entitled for the company's insurance amount. 

- As per Indian law , the insurance amount can be claimed by you , son and your mother-in-law equally, even you are a nominee in the said insurance. 

- However being nominee in the Insurance , you have first right to claim the amount .

The nominee acts only as a trustee on behalf of all the legal heirs for receiving payments due to the deceased and cannot claim the entire inheritance legally even if he/she be also a legal heir. In your case, your late husband's insurance proceeds shall devolve equally upon you, your son and your mother-in-law. Your father-in-law shall be excluded as he is not a Class-I legal heir.

They will have share as per Hindu succession law one part will go to them

Dear client The distribution of the insurance claim depends on the terms of the insurance policy and the laws of the country in which the policy was purchased. If your husband designated you as the sole beneficiary of the policy, then the insurance company should pay the full amount to you. However, it's possible that under the law of the UK, your father-in-law and mother-in-law may have a right to make a claim on the insurance proceeds. It's advisable to consult a lawyer familiar with the laws of the UK to determine your legal rights and obligations