I think you will find that there are dimples in the boot floor where the recommended Vauxhall fixing points are, and it's just a matter of drilling in the dimple. Some say you should remove the sound deadening material around the spreader washer, other say you it isn't necessary, although it will flatten and possibly split it around the washer, but equally it also should help with sealing the bolt head and washer to the floor.
Most towbar manufacturers use the recommended Vauxhall fixing points; some others don't, in which case you won't be able to use the dimples as an easy reference, and will have to ensure you are drilling holes in the right place for your particular towbar.
The other issue I have had with towbar on Elites is in relation to the parking sensors. The towball was the fixed flange type, and when in reverse, the sensors picked up the towball and sent the sensors haywire. However, the wiring was an aftermarket and scotch locked into the wiring by the rear light.
Maybe others could confirm, but I understand some Omegas are fitted with a take off already in the loom in the rear panel for the genuine VX towbar wiring kit - is this simply a 'plug and play'? and does this isolate the parking sensors when connected upto the trailer /caravan?
But how do you get round the sensors picking up the towbar when solo and engaging reverse, other than having a detachable swan neck type of tow bar?