That is correct. SCVMM Workgroup Edition can manage up to 5 physical hosts. Pricing wise then you'd be looking at around £100 (but much less if you're doing it under School Agreement).
Am I right in thinking the SCVMM component is available separately if all you want to do is manage your Hyper-V servers? I'm looking at starting an Hyper-V role out later this year and was considering this for next year. I had it in my mind (can't remember where from) that I could get a SCVMM 2008 Workgroup Edition for about £500 to cover the 4 or 5 Hyper-V servers we'll eventually have?