Hi Kimdo - your best bet is to get a 3GB mobile broadband SIM card pack from an online merchant - e.g.
New 3GB Mobile Broadband Preloaded Data Sim Card for: Amazon.co.uk: Electronics
You then get a SIM card which is valid for 3 GB/3 months - the clock starts as soon as you use the SIM card...
The cost is £12.45.
No registration or activation is needed on the Three network - APN details etc are in the SIM pack.
I use these in my Novatel MiFi - every three months I slot in a new SIM card - it just works.
Hope this info helps.
Steve
[size=2]Steve Gold
PO Box 1014, Sheffield S10 5YG, UK
Home mobie: Telefonica O2
Other UK mobiles: 3, Vodafone, Virgin
Foreign SIMs: Toggle (multi); Germany (Fonic); Poland (Orange PL);
Skype: stevewgold