Hedra are one of the few 'Public Sector' consultancies, they are pretty good and have a string brand in that industry.
Also Deloitte, Dettica, Avail, and E&J BAS have a name in this space. Serco do a lot of public contract management (but not a lot of consulting).
For a list, have a look at Catalist, the Government Procurement Framework for buying Professional Services. This should be available from the OGC website. This will show you who the government view as public sector consultancies - though this might not count for much ;-)