Hi I recently got a V166 which I am using in bridge mode with a descent wifi router and upgrade to the latest firmware for BT.
I VDSL provider is Bt Halo 2 (80/2) and with BT hub 6 I was always getting 80/20mbps.

Initially, with I was syncing at 76-77mb which is acceptable but after a few weeks it was down to 60mbps and the real download speeds were in the range of 54-55mbps.

I assumed that there was an issue with my line and called BT and after a line reset my speed was back to 80mbps, however switching back to 166V the connection dropped to 73 and after week down to 62mbps and looks like it will deteriorate further.
If I plug in the BT hub it does go up to 67mbps and will probably increase over time.

Is this product rubbish with BT, did I get a faulty modem or am I doing something wrong from my end?

If the SRA is dropping the line rate while the BT Hub 2 keeps a steady line speed, that's clearly a problem and not something that should be happening if the other modem is keeping a higher speed.

Please get in touch with the support team, there is likely to be a compatibility issue over time with your street cabinet's MSAN. The output of "vdsl inventory cpe" and "vdsl inventory co" would be useful, along with "vdsl status" and "vdsl status more" from the CLI or web console.
I think getting the output when the line rate is higher, then the same output when it's lower would be great to compare, to determine whether it's something wrong with the target SNR it's using or some other issue.

Thanks, will contact support and share screenshots with all the vdsl stats you suggested.