You're right. The cutoff time makes no difference on Fridays because there are no Saturday or Sunday deliveries, so nothing can be delivered before Monday anyway. Nevertheless, you ought to start your order today in case Ratesetter decides to manually consider the loan application that's necessary in order to get the phone on a pay-monthly basis.
Another think to check: make sure you're logged in to your account when you order the phone otherwise you'll be asked to buy a goodybag along with the phone. That goodybag will then be on a different account, with a different phone number, so if you carry on using your present sim you'll have a goodybag that you can't get at. Log in before ordering and you don't need to order a goodybag.
Incidentally, you will get a new sim in the box but you simply keep it as a spare for future use. Though you might need it if the new and old phones use different sized sims -- in which case you must log in to the old account when you active the new sim, in order to keep the same phone number. More information about that at https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084