Not sure what to say here
I think this is a showcase for not swapping and changing cars too often. Get one up to scratch and keep it, much cheaper in the long term.
Taking it to another garage was not a good idea really, you need to take it back to Halfords as they did first MOT. They would have no wriggle room on failing it on further items, as they saw it previously.
Not sure I would use your local garage either, as it's a case of them taking you for a ride or they are used to very new cars. I've never heard of an Omega failing for rear sub-frame bushes, mine on the 2.2 were shot. Original at 15 years old, but never as much as a warning on MOT's - Same can be said for our 3 other Omega's at the time!
You either need to take this all on and fix it yourself, as the cost of a garage to do this would be far too much. Looking at the most recent fail from your local...
1) Brake Pipes, I'd clean this up with a wirebush as often the corrosion is on the surface, then put some grease on them, this should resolve these issues. = £<5
2) Brake Discs, well other MOT said balance issue, so a new set of front discs would be a good investment. = £80 Approx for set of GM's on TC with Pads. (Cheaper ones can be had if wished)
3) Wiper Blades, Easy fix = £10-15
4) Rear Sub-Frame Bush - First I've heard of one failing personally (could be wrong) - Easy fix though = £50 for the pair
http://www.ebay.co.uk/itm/VAUXHALL-OMEGA-REAR-CROSS-MEMBER-FRONT-BUSH-/330439576840?hash=item4cefbbcd085) Sub Frame Mounting - Wire brush and hammerite time = £10
6) Seat Belt Anchors - Needs closer inspection, is it surface rust? If so same as above applies using the same tin! = £0
7) Anti Roll Bars - Replace the pair, dead easy job = £25
http://www.ebay.co.uk/itm/MEYLE-Front-Left-Stabiliser-anti-roll-bar-DROP-LINK-ROD-Part-No-616-060-5575-HD-/351180970554?fits=Model%3AOmega+B&hash=item51c4045a3a:g:X4IAAOSwQItUJZxiTo fix it all up, you would need less than £200 notes, could all be done for £150 quite easily.