I've never been in contact with these types of firms. No money worth mentioning in my family.
These firms will take a cut of whatever the amount is available, possibly at least 20%, maybe more. Not really sure about that.
You also have to consider how many other putative inheritors there are and how big the estate is.
If you are contacted by any of these firms you would do well to find out who the deceased is and see if you can find that person on the bona vacantia list.
https://www.bonavacantialist.co.uk/This should say what the estate is worth. I think you can apply yourself without going through one of these firms, but it is not an area with which I have any great knowledge.