It doesn't matter if you think it is not a complex feature or not or whether you think BT could add it if they really wanted to, the bottom line is the BT hubs don't have it so if you want QOS you will need to buy a different router.
Quality of service is not a complex feature, it could be patched into current hubs if they really wanted to
There was QOS code present in the Home hub 1, to allow the old BT Broadband Talk service (VOIP) and the original BT Vision service, to get upload priority.
Download QOS over the core network has to be applied at the source where it enters the ATM (Asynchronous transfer mode) port . This is done on BT YouView and was done on the old BBT service, where the ATM frames are "tagged" as ABR (Available Bit Rate)
Routers that have the facility to apply QOS and bandwidth control, can only work on the internal bridge between the WAN and LAN, they cannot control the priority over the core network, all they can do is to give priority to certain devices on your local network.
Its not a perfect solution, and some delay can still be experienced on critical applications like games, but it does help where you have users streaming video, or uploading files.
Imagine the amount of work BT support would have to do to determine where the delay is occurring, especially on a cheap domestic service.
Some business services do guarantee QOS for certain applications, but you are talking about thousands of pounds a month rental.
The TP Link router does a reasonable job of prioritising traffic. One of our mobile phones has WiFi calling, and that is given the highest priority to ensure perfect call quality.