Hey,
For the UAI, your best 10 units will always be counted. So for example, with your subjects:
3unit maths, pdh, eco, business, adv eng
Lets assume Ext 1 maths is your worst unit. In this case, this would be the one unit that isn't counted towards your UAI.
Conversely, if say, buisness is your worst subject, only one unit of buisness would count (or half the subject value).
So your subjects won't be split into one unit components as such, its just if a two-unit subject is your worst subject, one unit will still have to count to make up the full 10 units.
Hope that this makes sense.