Tell your insurance, they will sort it,as stated above.
but what a muppet, ifi had rear ended someone and they offered me to pay in cash i’d have bitten there hands of for £300, and no whiplash claim.
My other half went into the back of someone 2 years ago the damage cost arounds £200 plus admin,with out whiplash claim. and we must have payed around £1000 extra on insurance because of it.
The down side is if your OH does have and health issues long term as of it you would have no help from the insurers down the line.