people claim that these sites are scams
I would like to know which idiots or which website says so. And I would say you haven't done your research properly.
At the bottom of the page you can see this, on IG's website, very important quote
authorised and regulated by the Financial Conduct Authority
Plus500UK Ltd is authorised and regulated by the Financial Conduct Authority
Secondly you go and check the authencity of the firm in question on this link.
your money in, you might as well kiss it goodbye
This might be true because they trade in products, which if you don't understand you will more or less will loose your money.
N.B. I have an account with IG for the past 5 years and they haven't scammed me yet.