Lizz is correct regarding the terms and conditions, working hours etc etc. They really are shocling, BUT its a very interesting environment to work in and I certainly get a lot of job satisfacttion.
What 750XL means is that the airlines will hire people who have worked on their product whilst employed by the handling agent, and as a result they know are trustworthy and capable of doing the joib required. Virtualy all the staff I have encountered who are employed by an airline have been at a handling agent in the first instance.
It really is a who you know industry.
Can I ask why you want to volunteer when its likely you would get a paid position with a handling agent? They will start recruiting around the end of february so look out for advertisements then in the local papers and the directgov job centre website.
Regarding the check in agent, I can understand her gripes, although its no excuse for poor service. She will be getting paid not much more than minimum wage to get out her bed at silly o'clock in the morning and take all sorts of grief from folk, both colleagues, managers and passengers (not aimed at yourself and family btw).
If you would rather miss out the handling agent altogether then perhaps cabin crew is a route you should look at if you are desperate to work for an airline, and then look to go into a ground role later on.