I’ve never had to deal with the engineers but do own an insurance brokers and have helped arrange the survey for clients in your situation.
Has been money well spent for some and money wasted for others and you’ll not know until you receive the report.
Often however, we have got an insurer to give an indication if they would be willing to include subs provided we had the original engineers report, monitoring results following remedial works.
What was the underpinning for and how long ago?
If you were willing to buy the house in the knowledge you wouldn’t have subsidence cover, it’s reasonable to assume someone else will. The price will reflect this.