Don't think the Openreach will not replaced new cable to shortern the line length. I did ask the engineer how can I get my G.fast full rate 330/50 as he told me they can arranged a new re-route cable to get below 150m away from the cabinet but has warned me it very unlikely to do this because it will involved wayleave grant premission, cable length cost, labour work cost plus vat will be very expensive bill. I think your BT Hub are better to use it for G.fast than the Openreach Modem. But they did find the fault at the port swap to gain extra speed on my G.fast from there as the pair was shared with other property which it shouldn't have happen.
My G.fast only get 181/28 on Openreach Modem but my zyxel gfast router give me better sync higher than then the openreach modem. You could try this G.fast filter but it discontinued as it pretty good as it gained 7-9Meg extra as I got one of it from here:
https://www.run-it-direct.co.uk/adsl-vdsl-filters/gfast-plug-in-filter/