If you go through an umbrella company you become an employee of that company, giving you things like paid holidays and statutory sick pay. However you'll get clobbered for two lots of National Insurance contribution because, as well as paying your own you'll end up paying the employer's contributions as well.
Take a look at this link. (The first bit is about supply teaching, so you might not think that it's relevant to you, but if you scroll down you'll come to stuff about the construction industry):
https://www.theguardian.com/money/2016/oct/21/temporary-workers-umbrella-companies-extra-costs-dodging-ni-cutting-rights-supply-teachers
See also here:
https://www.bbc.co.uk/news/uk-england-39058166
I note that Apex (whose site you've linked to) doesn't want to provide any figures. One of their rivals, Parasol, offers a calculator:
https://www.parasolgroup.co.uk/
(You'll see that, as well as showing you what you'd end up earning for a 37½ hour week through their umbrella company they also show you what you'd get if you set up a limited company but with their sister company doing all the administration for you. That's another possibility which you might want to consider).
If it was me I'd want to hang on to as much of my money as possible and go down the self-employed route (even at the expense of losing out on paid holidays, etc).