I would like to apply for super priority so that I can get things sorted out quickly.
Is this a different page/website? Or will I be given that option I have paid 2400 GBP?
Super Priority can only be purchased at the same time as submitting and paying for the application - it's all done in the same transaction on the same page.
However, there are only a limited number of Super Priority slots released each day (I think around 50), so if you are not given the option to add Super Priority for £800 to the £2400 amount when you submit the application, that means that all the Super Priority slots are gone for the day... and so you will need to log out without paying the £2400 and try again another day.
The Super Priority slots are released at 1am each night, so if you log into the application for 1am, you may be able to get one... and if not, you can just keep trying each night until you get one.
You cannot add Super Priority at a later date after submitting the application and paying for the visa, so if you definitely want to use Super Priority, just make sure you don't pay for anything until the application submission page gives you the option to add it.
If you have already paid the £2,400, unfortunately, you cannot now add Super Priority... so your only options would be:
1) wait up to 6 months for standard processing
2) withdraw the application before attending biometrics and apply for a refund of the £2400 (this can take a few weeks to be processed). Then start over with a new application form and pay again, this time adding the £800 for Super Priority if you can get a slot