Are the parents in law UK nationals?
Assuming they are not UK nationals (I didn't ask about residency as you told me they lived abroad) and the gift is made from assets held outside the UK, there will be no Inheritance Tax implications for your in laws when they die.
UK Inheritance Tax is not normally paid by the recipient of a gift or legacy so you won't have any IHT or other taxes to pay in the UK on the gift. It can only be paid by the recipient if the deceased estate does not have sufficient funds to pay it on a gift made in the seven years before the death of the donor. That won't apply to your in laws unless the gift is made from UK held assets which exclude cash held in UK banks in a currency other than British Pounds.
I hope this helps but let me know if you have any further questions.
You don't have to disclose the gift in your tax return. However, you should be able to prove where it came from if asked by HMRC. A letter from your in laws should suffice.
You might also give advance notice to your bank to avoid their money laundering procedures kicking in especially if it will be paid to you electronically.