Dee, we always buy our tickets before we go. You can only get the 10 day world pass in this country (Virgin Travel Store I believe are doing it most cheaply, I think it's £186, could be wrong!)
However if you want longer than 10 days access, the 'fun in the sun' tickets are great value at about £200/adult, £161/child (at $1.55/£)
BiGGys right, buying Universal tickets direct seems to be a good idea, unless the 4 or 5 day flex tickets appeals. We've just bought 3 power passes from Universal for $99 each (£63.87), which gives us a whole year of access (with a few blackout days) and 1 preferred annual pass for $149.95 (£96.74) which gives us free parking, waahey!!!
Ian