The line "recessuros ut divus iulius recessisset modo virtutem maiorum suorum aemularentur" has me puzzled.
The Brittani are speaking.
They (The Romans) would withdraw, as divus iulius had withdrew, if only they(the Romans) rival the valour of their ancestors.
This seems off to me, unless it's sarcasm?
aemularemur would make more sense to me, giving; if only we (the Brittani) rival the valour of our ancestors.
This is what the translator of this http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0081:chapter=15 has got, whether he had a different text to what I've got or emended it himself I know not.