The trouble with these rust proofing / preventative firms is, in my experience, that they don't know your car as well as you do. By that, I mean that you know all the areas on the Omega that are prone to corrosion, and unless you are breathing down their neck whilst they are doing the job, can you be sure that all the likely areas have been treated?
My opinion with the Omega is that that most areas where corrosion starts so as to cause a problem is generally from within, rather externally, and knowing where to start is a job for a nice sunny day. If you search rusty wheel arches for example, you will find info I posted how best to tackle this one aspect of rust prevention.
It reminds me of the hype back in the 70's and 80's about the likes of Ziebart, and how it prevented internal corrosion. The number of cars I used to see several years on with the telltale yellow plugs, yet the cars were rotten as a pear purely because the treatment hadn't been applied in the correct rust prone areas....
If I were you, I'd do it myself - plenty of places will steam clean the car underneath first, and it's got to dry throughly afterwards, but then it's worth getting grubby for and at least you would know it had been done properly...