Oh I'm so sorry to hear how serious the trauma is.
Unfortunately when the injury is this severe, we cannot choose the cheaper option, which would be amputation of the affected limb.
Although, it may be that the more badly broken leg could be amputated (this is a much cheaper option than fracture repair) and that the other leg can be repaired.
£12,000 is a very steep quote indeed and I'm assume this includes specialist involvement and MRI scans.
I'd absolutely be seeking a second opinion as I'm confident you'll get a cheaper quote elsewhere.
We can see if there are any vets who will allow a payment plan; this would be at their discretion.
There are also some vets who will allow payment on credit.
Another option is to crowd fund with e.g. a Go Fund Me page.
If the fractures are complex and surgery may not work or is simply not affordable, we may consider euthanasia. This is to prevent suffering and long-term pain and mobility issues.