What sort of tyre mileage is expected with our cars 
This is vastly down to the tyre and type of driver, personally I'd be very upset with kind of miles TB is quoting here.
48k from a set of Goodyears I had, 35k from a set of Falkens. Currently approx 5k on a set of Kumo's 
Tunnie, how you actually arrive at a destination without pressing any pedals and getting out and pushing the car round corners is beyond me.

...its not bloody sailing.

... if the car suspension remains flat and unchallenged then the camber won't deflect and you will get more even wear out of the tyres. However the sailing driving style is no guide on average tyre wear.
Op, an absolute minimum of 10k from A bad/non grippy tyre driven by a "progressive" driver/15-20k from a good grippy tyre on a well set up car by the same driver. 20k shouldn't be too difficult generally. More if driven more sedately, as Tunnie says to be fair.
Your not likely to find a company that sets camber, that isn't capable of full geo. Don't skimp. Proper job WILL see a return in decent tyre life.
You'll also find the car is more accurate, stable, safer and easier to drive giving a much better experience.
Spings sag with age and use. This means average camber position will increase as the ride height drops slightly. Add in the fact most garages don't know the omega is adjustable for camber when they tighten the shock bolts, which set camber, AND the fact that Vauxhall settings and production assembley was hardly accurate from new, its best to set the car up correctly as soon as possible after taking ownership.
This applies to most reasonably sized cars, not just the omega.
See wheels in motion web site. They have franchises around the country, and generally speaking are the only company that have settings that work. There are others, bit I'm not aware of any others that are near your location.