Hi benjy,
If you look at the current UKBA listing of approved tests you'll see that IELTS tests have a minimum passing level at B1
Just as an aside, UKBA require a pass at level A1 (which is actually lower than B1)

The passing grades are shown as:-
Listening – 4.0
Speaking – 4.0
Reading – 4.0
Writing – 4.0

UKBA are only concerned with listening & Speaking