A big 4 accounting, or legal firm?
London makes a big difference. I would guess that turnover rates in London are much higher than the regional offices; doens't mean that London staff are at the centre of the universe, but does mean that HR in London will be more on their toes when it comes to annual promotions and the like - they need to be, in order to stop people moving to competitors. Regionally, things just don't need to have the same drive, so they generally go slower. That is nothing to do with you and you cannot influence it, but you just have to accept it and work out how to deal with it.
You have to discount the 12 years younger thing as well. Not just because this is now the law (you rate/promote totally on ability to do job and suchlike - it doesn't matter how many years experience you have) but because promotion is all about the ability to do the next job up, and not about the ability to do the current job. Your experience does count to you and shoudl be a factor, but you want to be rated on your abilities and not just on the fact you have 'done the time'.
You also need to put away all the stuff about your previous roles: they are the reason that the company wanted to hire you in the first place but are now less relevant. Try to trust in karma that the people who messed up your previous promotions will get their just dues in some way, just leave all that annoyance behind (I have been there myself, I do sympathise, but you really need to accept that all that went on before doesn't matter).
So, where you are is in a new job, where you have completed 12 months employment. You have done a great job, and you know you are ready to go up. Other people are going up and you need to make that happen.
And where is the firm in all of this? They like you obviously, but don't know you that well. And your new boss has taken over a team and needs to perform well in order to make sure he doesn't get fired like his predecessor (Sp?). The last thing your boss wants to do is rock the boat by going out on a limb for you I guess. What's in it for him? If he keeps you in the current role then you will be an outperforming manager, which reflects well on him. If he tries to get you promoted your cost to the firm goes up and he gets no benefit from it himself. In an ideal world we all have great bosses, but here in the real world there are many many bosses who want to keep the best peple around them and they can only do that by holding them donw and not letting them bubble up to the top. It's a crap strategy and doesn't work but I have seen it loads of times. And in an environment like we have at the moment, even good bosses are a bit scared of putting their heads over the parapet to support their staff at the moment.
What to do about it? Fortunately, if it is a big professional firm, there are plenty of avenues. Legally the firm has to treat people fairly, and if it is a large org it knows the law (might not follow it all the time, mind you). Treating peple fairly doesn;t always mean mean treating them equally, though, and a promotion freeze in the regions but not London would be fair (so long as all the people in the same level and same office as you are affected by this). A simple question into HR will establish if something like this is the root cause (if so, nothing really you can do, TBH). Don't ask your boss because he may say this is the case even if it isn't and you will have put yourself in a cul de sac as you will then be 'crossing' him if you go to HR to double-check.
If there is no such freeze then you need to ask for a meeting with your direct boss, the partner in charge of your area, or HR. your call on who you talk to. Set out clearly that you have excellent experience, were led to believe when you were hired that you would be promoted witin the year, and now that you are seeing your colleagues being promoted around you, you are struggling to see what it is you are doing wrong, given that your colleagues all agree you are doing a great job and your ratings are outstanding. Could they please explain what it is that you need to change to achieve the same promotion that is being handed out to others? I think that approach is not so assertive and puts the ball in their court. And then see what they say.