>> SNR Margin (Downstream/Upstream): 15/22
With adsl its mostly the downstream figures that we need concentrate on.
With max by default the Target SNR is 6dB, but if your line has trouble maintaining stability then the DLM can change this. Configurable options are 6,9,12 and 15dB.
Judging from your stats it looks possible that your Target SNR has been set at 15dB to keep your line stable. This would tie in with the fact that at one point you could get 4.1Mb but now can only get 2.1Mb. The higher your Target SNR then the lower the speeds at which you are going to sync to the exchange at.
Target SNR and SNR Margin although related are not the same thing.
You must have been having problems with stability at the higher speeds for the DLM to set your target SNR to 15dB.
If UKO are using adsl2+ then it may be able to squeeze a bit more out of your line than on max - but I seriously doubt whether it will be that much more.
You may find yourself in a similar position as to what happened on max - you start off high, but speeds have to be lowered with a higher target SNR to maintain stability.
The main benefit of LLU is that there is no IP profile to contend with, and you get more or less the speed you sync at (after allowing for overheads).
[edit]
James has posted a bit more info too whilst I was typing out this reply