You've entered LEL? Excellent! I'll be volunteering at the Pocklington control so if you want an extra unofficial bag drop, let me know.
I bought my Etrex for exactly that reason: it took AA batteries rather than the inbuilt ones used by the Edge series (then the 705 et al). The Etrex 10/20/30 replace the Etrex Legend/Vista and as such, they also take AA batteries. It's definitely worthwhile getting one of the newer models, as they're more powerful machines.
By the way, if you want some training for LEL, have you seen the
600km audax running from York station on June 1st? You might know the organiser Aidan from the Wigginton audaxes - he was riding a trike this year - and he sounds like he's got the event very well in hand.