Khidhr - I've covered in great depth in one of my editorials for "The Definitive Guide to UK Consulting Firms" the stages in your career where it's possible to transition into consulting. This will be available for download on the site within the next week, but for now I'll just start by saying that "tak(ing) a salary drop and start(ing) from scratch if completely necessary" is a non-starter. Firms have very defined hiring profiles and taking graduates with several years' experience to start in their graduate entry programme just doesn't happen. So you're looking at being hired as an experienced hire or MBA entry candidate, or not at all.
In my experience it's extremely rare for the top strategy firms to hire someone with your current profile. At experienced hire level they will be looking to bring in people with detailed sectoral or functional expertise - and preferably prior consulting experience. You might be able to move from Deloitte's consulting arm into an MBB - if you can get yourself into consulting at Deloitte - but extremely unlikely straight from audit. Or if that's not possible then completing an MBA from a top top school (I'm talking INSEAD, LBS, Harvard, etc.) will give you a shot at getting in.
To validate my opinion above, I've just been to LinkedIn and searched for people currently at McKinsey in the UK who also have Deloitte in their profile. You will see if you do the same thing that there's limited movement between the two - and those that have made the transition have all moved from Deloitte's Consulting business.
Not necessarily what you'd want to hear I know, but hopefully this gives you a more realistic picture of the options open to you. Do look out for the download of the guide (the 2011 edition, not the older one currently available on the site) in the next week as there's a lot more discussion of this contained within.