It depends. When they calculate UAI the BOS or whoever make all your class into 1 unit. EG: all standard classes are worth 2units and ext's are worth 1unit, as you know. So if you look at the rubric you'll find the ext's are out of 50 instead of 100. So, all 2unit subjects get scaled back to a mark out of 50. IE: 98/100 = 49/50. Advanced and Standard English's are the only classes that count no matter what. And you're probably aware that a mark of 40/50 in ext english is worth more then 40/50 in standard english. This is where scaling up of harder classes comes in. This may mean that a 40/50 in ext english becomes a 46/50 overall. Therefore, it's most likely that your ext english mark will be included in your UAI calculation even if it is your "lowest" mark.
Hope that helps.
Sorry about the length.
3U Maths, 3U English, Physics, ITVET (doing for the cert)