If it works then that's good enough for the MOT which is all you need to comply with. It's up to the manufacturers to get the correct brightness from the lights they install, not the end user. All the MOT requires is that every brake light fitted works, which yours still does.
That said (and without trying to lecture), given how important the HLBL is and the relative lack of visibility in the Zed anyway I don't think tinting any of the glass is a particularly good idea. If all your brake lights (including the main ones) had seriously reduced brightness then there's an argument that the police could prosecute you for not having a vehicle fit for use on the road, which is why I try and talk anyone out of putting that horrid film on their rear lights for the sake of trying to be cool. Well, that and it looks bloody vile as well!