If all you have available is ADSL Max, there will be a great deal of commonality whichever supplier you go with. The supplier will have to use the BT Wholesale DSLAM with its associated DLM (which are important factors in determining the sync speed), also the BT Wholesale backhaul. There will be some differences depending on how the traffic is handed over to the ISP and the geographical location(s) of that handover (which are the main factors affecting latency).
Unfortunately 20CN backhaul capacity is expensive and people are increasingly migrating away from this system as the roll out of newer technologies continues. Providers face difficult decisions about capacity versus visible contention (with contention causing jitter, noticeable slowdowns and, in more extreme cases, packet loss).
I believe Virgin, Sky and TalkTalk have stopped offering new 'off net' connections via BT Wholesale - these services, even if still available, have had a fair number of contention related complaints.
One important factor is whether you require static IP, or whether dynamic is acceptable.
It sounds like any of the quality BT Wholesale based providers can potentially meet your needs, but unlimited options are rare and possibly non existent because of the high backhaul costs to the provider on 20CN. AAISP do not do unlimited other than on their expensive dedicated bandwidth products (Ethernet over FTTx and so on). I'm fairly sure Zen's unlimited broadband product is only available over 21CN or Zen's own network, though you could ask Zen sales (I think they will only offer you one of their more expensive and limited '8000' products). I don't know if any of the other smaller providers could help, such as IDnet. Plusnet is a possibility, but their user forum has a lot of disquiet at the moment and they typically charge more on a Market A exchange.
Hopefully someone in a similar position to you will be able to offer some advice. I'm in a fairly small town, which meant we were not top of the queue for any upgrades. We had what is now Virgin cable in most of the town before there was any ADSL and, over the years, we have been fortunate enough to get 20CN ADSL, 21CN, a couple of LLU providers and FTTC via the commercial rollout to almost all cabs (a couple more were enabled via the local BDUK project). Hopefully, in time, there is some sort of upgrade available to you, but I realise you are after something for now.
Edited by David_W (Wed 14-Oct-15 08:41:09)