A question for British members

I have a friend who has been in the UK buying goods that is part of their business to sell to customers in US.
They did their business and were ready to leave but were then told by the customs office that they need to show another 2,000 pounds to the office before they will allow them to fly.
According to this friend, the customs said that since they were doing business in the UK and filled out the paperwork when they arrived saying the amount they would be spending turned out to not be the amount they spent they have to show the office this extra money. They don't have to pay them the money just show it to them.
My question is if any of you have ever heard of this before and, if you have, if you have any other insights into this situation?
I'd be surprised if you do have any answers since it seems to affect people coming into the UK and not citizens of the UK, but if you do have any info I will be very appreciative.
Thank you.
They did their business and were ready to leave but were then told by the customs office that they need to show another 2,000 pounds to the office before they will allow them to fly.
According to this friend, the customs said that since they were doing business in the UK and filled out the paperwork when they arrived saying the amount they would be spending turned out to not be the amount they spent they have to show the office this extra money. They don't have to pay them the money just show it to them.
My question is if any of you have ever heard of this before and, if you have, if you have any other insights into this situation?
I'd be surprised if you do have any answers since it seems to affect people coming into the UK and not citizens of the UK, but if you do have any info I will be very appreciative.
Thank you.