I would suggest the following......
replace all four discs with genuine ATE ones bought from Euro Car Parts, or failing that with genuine Vauxhall ones. As OE, these cars were fitted with ATE.
adjust the pedal using the clevis pin and brake lamp switch. The clevis pin that attaches to the top of the pedal is adjustable, and pretty much always needs to be moved to eliminate slop in the pedal. Bear in mind that the brake lamp switch controls the "off" position, so you can pretty much choose the brake pedal position at rest to suit yourself. Don't forget to tighten the locknut on completion.
you will be surprised how much difference to pedal feel this makes. Omega B has just about the worst pedal feel of any car I've driven, so any set-up improvement makes huge differences.
I would not recommend.....
going for "improved" cross-drilled or grooved discs, they're nothing more than a fashion statement.
fitting the vented rear set-up to your car. It's designed to work with a different master cylinder AND EBD (electronic brake distribution) The rear piston sizes are larger, too, so it'll make an already rear-biased brake set-up worse.