Also as a TT ADSL2+ user, I can say that I am fairly happy with the service. Perfectly reasonable prices and to be honest I've found the support adequate (I've had to use it many times in the past year or so due to a very odd fault), despite the many reviews you can find. I've never really used their phone support so can't comment there, however their forums are fine and frustration free, though perhaps a little slow. They effectively act as a ticket system which some other ISPs use. I can also confirm that I experience no congestion issues (though perhaps wouldn't anyway on ADSL due to the locally reduced speeds anyway).
With regard to the more technical points you ask, I believe I can provide some insight:
Options looks like they are Sky LLU, Talktalk LLU or BT ADSL2+
As I'm sure you are aware, you will have the choice of not only those three CPs, but also those using TalkTalk wholesale or BT Wholesale.
Can you still tweak the SNR margin on these?
As I understand it, the SNRM tweak operates during the channel analysis stage of the synchronisation where the modem responds to the exchange DSLAM with (among other things) the available SNR for each frequency channel. The tweak works by your modem lying to the DSLAM, saying the SNR is greater (for Broadcom tweak values less than 100) or lower (for tweak values above 100) than it really is. This means that for a 6dB target with a tweak value of 50, your modem will tell the DSLAM that the SNRM is 6dB when it is only 3dB in reality. Naturally, there is nothing an ISP can do to prevent this (with a small caveat as detailed below) as from their end it just looks like the 6dB (or whatever) SNRM originally aimed for. As such, you should be able to do this with any ADSL service.
I recall TalkTalk now have fairly aggressive DLM on their ADSL lines now
In early 2017 TT introduced a new DLM system for their ADSL lines from ASSIA, rolling it out exchange by exchange. I am fairly sure it is now in use on all TT LLU exchanges. Under the old system there were a set of published line profiles that DLM would move you between, and you could ask support to put you on a particular one either on a temporary basis or with the DLM disabled so you never changed profile again. I read conflicting reports as to whether or not it can be disabled entirely any more. Certainly the OCEs on the forum for TT retail tell users it can't be, but it may be the case that users that previously had it disabled continue to be in that situation (e.g. burakkucat). I have also heard comments that connections wholesaled through TT Business can have it disabled or fixed at certain profiles, and this page
https://support.aa.net.uk/TalkTalk_Wholesale_Line_Profiles strongly suggests this is the case.
There seems to be very little information about how the new DLM system behaves for TT retail, but I can give you my observations. Firstly it can be reset without any problems with a so called "connection optimisation" by the OCEs on the forum. I have never seen my connection operate in fastpath mode, and I usually see quite high interleave depths. My interleave depth is currently 64, but I have seen it as high as 512 following a fault. The other thing it does is after a reset, it learns the fastest speed your connection operates at and sets that as a cap. This is the source of the caveat I mentioned earlier, as if you get the DLM reset with your modem set to no SNRM tweak, then your line will sync at the normal 6 dB target rate which will then be set as a cap. Subsequently applying a tweak down to 3dB will then do nothing as you will hit the cap. To get the extra speed you have to apply the 3 dB tweak then request a DLM reset (this sped my connection up from 16.0Mbps to 19.7Mbps). The TT DLM never applies a target SNRM of 3dB itself, this can only be done using the tweak on Broadcom modems. As far as I know though both BT Wholesale and Sky do use the 3dB target, and sky even use G.INP (for clarity yes, this is G.INP on ADSL), so these may be more attractive for this reason.
This is probably a bit of an information overload for the question of which ISP, so in summary I'll just say that I think TT retail are fine with reasonable support, some minor DLM technicalities that can be worked around and there is no IPv6 connectivity if you care about that.
