OK, I'll will start with the brake lines... They've been advisories for the last two tests

Depends which lines they actually mean as the Omega has rigid lines that are attached to the rear trailing arms as well as the length of the car.
The wording suggests the trailing arm pipes.
The subframe mounting rust hasn't previously been mentioned, so you MIGHT get away with a gentle scrub with a wire brush and coating with underseal and a different tester. Obviously this doesn't actually fix the problem, but could get another year or so out of it.
The emissions suggestion is a thing... A 2.0 16v I owned previously failed in a similar fashion and the fix was a £60 system clean at VX... I was selling the car and wanted it to have a fresh Mot, which was carried out during the test drive. Paying to get it sorted there and then was the decent thing to do. Cost effective? Probably not, but it solved the issue.
Brake lines might cost you a couple of hours and dome materials to sort, but no big deal.
The rust, as already alluded, is potentially a different issue...