As @MSF suggests, one option is to click on the link on uSwitch (make sure you're logged out of your existing Smarty account first) then create a new Smarty account (with a different email address).
A second option is to log into your existing Smarty account, choose from one of the available tariffs, then chat to Smarty support and ask if they can switch you to the special offer you've seen on uSwitch when your plan renews (i.e. you won't be on the special offer tariff for your first month). This is not guaranteed, but in my experience they are usually happy to do this. But if you want to guarantee getting the uSwitch offer, just go the first route.
