PA Consulting are fairly strong in CG/public sector. They're independent of outsourcing operations etc.
Capita Consulting are one of the big players for the public sector. They get a lot of work on the back of their wider service offerings.
Serco are going into consulting, also on the back of extensive public sector operations.
Turner & Townsend do some PFI/PPP work I think, and are recruiting. Some of the accounting firms also do public sector work. PwC have a decent reputation for it.