PwC have an advisory services team - Infrastructure, Government & Utilities - that consult on PFI/PPP. RSM Robson Rhodes also have a team that I have heard (second hand) some good things about.
PFI/PPP is an area that a lot of firms have tried to jump into as part of the trend towards more public sector consulting. My personal view is that the quality and sustainability of these service offerings is very mixed. As the market settles down it will thin out - some of the players forced out of the consulting market will undoubtedly blame a shrinking PFI/PPP market. However, I foresee this being a growing niche albeit at a moderated pace and with tougher competition for consultants as customers become more discerning and educated.