It does depend on area and particular service, but it sounds like they're trying to be as quick as possible.
I was first referred by my GP sometime in January (2014) I think, they pushed it through as urgent and I saw the psychiatrist in February, then the Severe Eating Disorder Service a week or two after that, so it should happen pretty quick if you've been put through as urgent with a low BMI.
Remember that if you do need to in the meantime, you can go back to your GP, or you can ring and ask them to check the referral and how long it might be - they should be very happy to support you in the meantime.