There are no rate limits at all if you choose Plusnet Pro.
That's not what your traffic management
It clearly states, even with Plusnet Pro, the following services are "managed": -
(and it states "Managed - Traffic which will usually run at line speed, but may be limited in the busiest periods").
Not only that, by the time you add £5 for "pro" onto the £24.99/month Fibre Extra package, your customers might as well go for the 80Mbps BT offering (which is actually unlimited, rather than limited to 120GB a month, plus they'd get 20Mbps upload instead of 2Mbps).
You still don't lose out (as the money will go directly to BT, rather than to BT via Plusnet).
vDSL2 FTTC Infinity with BT
DL Sync 80Mbps
UL Sync 20Mbps