Hiya.
The £30k will be per month for 10Gb.
They support these by giving leased lines priority on their network, and ensuring that all leased lines are able to receive maximum bandwidth across their network at all times. Note that this is not the same as ensuring enough capacity is free for all private lines to run at 100% all the time. As long as leased lines never see congestion the operator is doing what they said they would. Most leased lines don't run even close to full pelt, especially those being used for Internet access, the usual suspects for that are links between data centres replicating traffic.
This is done via a combination of either dedicating or reserving backhaul, depending on the size of the link, and ensuring that the core network is never congested, alongside perhaps having QoS in place so that, if they lose a ton of capacity and end up in a congestion condition, the private lines get the bandwidth first.
Network operators will try and scale their networks so that, on the core, no link runs at 50% or higher utilisation. As long as they do this they can lose half of the capacity without customers seeing any performance impact.
The specifics of the leased lines themselves depend on who you purchase from. A private line over fibre will be dedicated back to the exchange. From there it'll terminate on an edge router and be transported over a shared network to wherever it needs to go, unless you are ordering a massive bandwidth line, in which case you'll go into an optical switch, not a router.
I would consider as a far cheaper solution using a router that can load balance and getting 2 x FTTC lines from different network operators. The usual options are TalkTalk Wholesale, Sky, Vodafone, BT Wholesale, possibly Zen.
Alongside those, if possible, a backup Virgin Media cable line, or even a backup 4G line, so that absolute worst case if your local cabinet is wiped out you've at least some access.