how do you get predicted 2:1 (or above) from first year

It depends on your specific university guidelines which are usually highlighted in the handbook but generally your credits have no real influence over your prediction i.e. you could have 100/100 credits but be averaging 45%. Your predicted classification is usually estimated on your performance so far - so if someone is scoring 70%+ on 8/10 modules then they will likely be predicted a 1.1. If you score an average of 60-70% on your modules then you will be predicted a 2.1 and so on.

It's usually just an average of your grades from all the modules. If you got a 2:1 on your first year, that's your predicted grade.