Having agonized over another 3G modem (posted last night) I found this on Amazon which is £20 or 25% less than it was a year ago when I bought mine.
I know preloaded SIMs are already pretty cheap on Amazon but this looks like a good way to get the modem if you haven't already.
Prolly not the bargain that was seen here on expansys, but when that runs out there's always this, no rush. Getting 12Gb with your modem, from 3, is a sure way to avoid any problems that may be encoutered by using SIMs marked 'not for sale separately'.
12 months start from when you first 'activate' your SIM. I waited 11 months and used cheap fleabay SIMs in that period :)