my english teacher told us that, if say for example, u were doing 11 units, and the 11th unit was ext 1 english, it would still count (regardless of the fact that ext 1 may have been ur worst unit - which should have then theoretically not been counted).
ok, let me try to explain myself. say u got 90 in adv english, and 30 in ext1 - ur not off the hook coz u did shit in ext 1. the avg of the 2 (i.e 40/50 - for each unit) is the mark considered for ur uai, so all ur english counts. i think this case is more valid for people doing ext2 english, to discourage people just bludging ext1 english, and devoting all their efforts to the the major work.
i hope this makes sense... but i guess all we can really do is try our very best! :S i dunno if we can outsmart the system
take care,
mary