I wasn't sure if this was possible on FTTC unless the ISP had something setup at its end?
That's only if you need line bonding, so it appears as 1 connection. You can simply buy 2 FTTC services and run 2 modems with a dual wan router. That would be exactly twice the price. Line bonding will cost considerably more than 2 x an FTTC line as it requires the ISP to have things setup at their end.
391 lines served by a 256 cabinet is not bad.
We had 877 lines served by a 256 line ECI till recently. They just added a 288 Huawei (expandable to 384). This doesn't count another 40 or so new properties still not showing on codelook.
The problem of capacity is worse with ECI cabinets as it requires a whole new cabinet. Huawei have been developing their hardware and have released larger line cards. Both the Huawei cabinets used by OpenReach can now be expanded a little for a fraction of the price of a whole new cabinet. If they had to add another small Huawei to your existing cabinet it would almost be a fibre port for every property. So that's probably unlikely as they also want to add/sell G.Fast.