Good advice Sla. If they didnt like what they saw, you wouldnt've got this far.
Just be yourself and it'll be fine.
With regards your age, current recruitement is taking a long (as always) time. It took 2.5 years for me to get in, from initial letter to start of course, so if you get cornered maybe play that one as a last resort.
With ref licence, I'm sure someone will correct me if Im wrong, but you have to be a minimum of 18 to get a student ATCO and a min of 20 to achieve your first validation? I think the info is in the ANO somewhere, but I dont have time to check.
All the best, be sure to let us know how you get on.
Bri.