if you are going to buy the express booklets then one day should be ample for IoA unless you want to go back and re-ride stuff(i always go back as i cant get enough of universal or disney) and a tuesday/wednesday should be fine
(EDIT One thing to bear in mind is the 5th and 6th are directly after july 4th which is a huge deal there so there maybe a greater proportion of people on vacation there. that goes for all parks not just universal). the days to really watch out for are weekends though with the schools out in the USA its still likely to be somewhat busy.
I had a quick browse around
www.univacations.com (the website that handles ticket bookings etc) and the phone number to call for express plus passes is 1-800-711-0080 so from england you'd have to ring 001 800 711 0080
the people on the end of that line will be able to book them for you and they should give you the option of picking them up from the ticketing/services window by the ticket turnstyles. Just make sure you:
1. tell them you want to pick them up.
2. take the card with you that you paid for them on.
3. take some other form of ID to prove your you (passport or photocard driving licence is preferable.)
Also the people on the end of that line will be able to tell you what days there are passes available for, for both parks etc.
Hope that helps. Any further questions just ask!