Min line speed (minimum line speed) is VERY DIFFERENT to your current problem.
With Virgin Media you have a fixed line speed based on the product sold.
NOTE: Not saying what Virgin is doing is right, but simply that there is a reason they can do it, there is no law saying they cannot
As with PON and FTTP cable connects at the full speed available to the modem, or to the cable segment, whichever is lower.
VM varies between 600Mb and 1.2Gb of downstream capacity. Will be at GPON levels downstream in the not too distant.
As I said elsewhere I strongly suspect OP was provisioned on an area using the legacy Cisco or Motorola equipment, limited to 600Mb. My own area's node, bits of which are newly enabled, is on that equipment pending upgrades and 300Mb is not available. I imagine they released as load was relatively low, and were planning upgrades to the newer equipment which have, at least in the case of the hubsite serving here, been delayed.
Those who may remember when BT Wholesale had a bunch of issues with performance a couple of years back, VM are in that situation much of the time when it comes to upgrades to relieve capacity problems, with the added bonus of having to replace equipment out in the field in a bunch of areas as they are running out of bandwidth at the RF level.
When that's done and the DOCSIS 3.1 equipment is in place things will be a ton better, it's just taking a really long time in a large part due to VM's legacy as a bunch of smaller companies.
When I worked for ntl there were some really bizarre sites hosting broadband routers. A converted public toilet and an outhouse near a pub come to mind immediately. Moving equipment around, handling the optics, and upgrading power and cooling in sites used originally for cable TV and now used for linear and on-demand TV, routers and transmission equipment, with a side order of CCAP
is painful. VM don't have the luxury of a load of additional space freed up by a migration from analogue telephony to digital switched.
it also brings factors outside of control Upgrading cooling requires planning permission, upgrading power relies on the local power company, running out of space requires extensions to be built or additional property to be acquired.
Worst part for VM is that at some point they'll be moving more and more equipment out of those sites and into cabinets in the field.
I'm not excusing anything that's happened, but just trying to add some colour. Relative to the situation VM are in the capacity planning and upgrade requirements on Openreach, BT Wholesale and individual ISPs are relatively trivial, there's loads more to go wrong and get wrong.