Not necessarily a scam, this is a legitimate source of work for many firms. And to be fair, some people get money that they would never have known about. However, if you can prove yourself that you are entitled to a share of an estate, you can do so without anyone else taking a cut.
Have a look at this Treasury site bonavacantia It tells you how to submit a claim. You need to show evidence of your relationship with the deceased.
You said some relatives have already signed up with this firm, so presumably you know the name of the relative. That's the hard bit - if someone rings out of the blue and obviously doesn't tell you the name of the distant relative, you need to find out. On the bonavacantia site you can search for names of people dying without a will. You can search by first letter of the surname or search by family name so you could try common names in your extended family. That probably wouldn't be much help if the relative had another name by marriage that you didn't know. You can also see the full list of names, in case you recognise one. The list shows the name, place of death and the amount of their estate. Then you just click on the name to submit your claim.
Actually, I've just had a look on the site, and there are only 63 names currently listed, so it's worth having a look at.
Under the FAQ, there is a bit saying I have cousins, do I have to share the money. And the answer is that once an entitled person is identified, the estate still has to be shared out under intestacy rules, although the Treasury Solicitor would no longer be involved at that stage. So that seems to imply that you would be entitled to a share now that your other relatives have been identified, even if you don't sign up with this heir-finding firm. I would still make a claim myself though!
Good luck!