Hi Rach - at my clinic they have 2 systems:
Anonymous egg donars and known egg donors
Anonymous donors just donate their eggs for whoever is at the top of the waiting list
Known donors are 'paired' with a specific person on the list (and that person then jumps to the top) A known donor can choose not to actually donate to their 'paired' person, but to the person at the top of the waiting list - and the paired person then jumps to the top of the queue
I guess the reasons for doing this are:
If the donor/recipient would feel weird about 'knowing'
If the donor is a relative of the male partner (obviously if bloky's sister is the donor, you wouldn't want to fertilise her eggs with his sperm!)
It's a lovely thing you're planning on doing and if you were closer to Birmingham I'd be jumping up and down now saying 'pick me' 'pick me'!