Hi Murf. To breed Lavender Cuckoos, If you take a Double factor Cuckoo Pekin male and mate him to a Lavender Pekin female, you will breed 100% Dark Cuckoo chicks that will all carry the Lavender gene. Select the best birds from this breeding and mate them together, and you will breed Dark Cuckoos, Blacks, Lavenders and Lavender Cuckoos!.
Theoretically, to breed Lavender Columbians ( Coronations? ) if you take a Columbian Pekin Male and mate him to a Lavender Pekin female, you will breed 100% black chicks, that will carry latent genes for the Columbian pattern and Lavender. Again, mate the best of these together, and amongst the many colours, and mis-marked off-colours you will hatch, there will be a small percentage of Lavender Columbians. These will probably not be well marked, but by back mating them to Good Columbian Pekins over successive generations, You should improve their markings, and produce more Lavender carriers, which, when in turn are mated together should produce better quality Lavender Columbians. As you say, it will be a long process!!!
It sounds as though your first hurdle will be obtaining some good quality Lavender Pekins! Try contacting the UK Pekin club, they have a breeders register. if you have no joy in finding birds closer to home, there are a number of Irish fanciers who regularly make the trip over to England to attend the big winter classic shows, which have Large sales classes, who may be able to purchase what youre looking for there and bring them back to you?