I understand to test PPP between two ethernet devices is straight forward without a DSLAM, but in our case most of the CPE devices they are configuring to ship to customers are integrated modems with the WAN port being ADSL/VDSL only, so without the DSLAM in the middle, you can't test, can you?
There is absolutely nothing user configurable that you can test on the Huawei DSLAM's that will also work in the live OpenReach network.
OpenReach have everything pretty locked down.
It isn't about playing with DSLAM settings to test that side of things, it's the CPEs we are concerned with.
The config on the Huawei is just about about getting close enough to what OpenReach use, without needing to be exactly the same (as you mentioned about the ADSL hardware), that we can do end-to-end testing on multiple CPEs without needing to have loads of actual OpenReach lines up to the office, with the added benefit of being able to test with the desired PPPoE login and force a slow sync speed if we want to test QoS, etc.
While those other configurable settings for the CPE are fairly straight forward, the CPEs we are shipping vary a lot between customers (no ideal, I know) and to have them arrive on site only to not work out of the box if our support engineers haven't set them up correctly is fairly embarrassing - arguably worth the investment in the DSLAM by itself, since centralized config generation doesn't seem like an option in all instances.
Having an environment for our engineers to learn on, end-to-end comfort test whole environments with before deployment and experiment with close emulations of real world scenarios has been invaluable in the past and it's something I'm confident that for the sake of a ~£600 investment will bring us a lot of benefits - although perhaps I am being naive, only time will tell