I use Spillers Layers Pellets from the mill but they tend to not be stocked by small feed merchants. I've also used BOCM Paul's and Marriages which aren't expensive compared with more specialist feed manufacturers and I'm sure equally good. Personally I think whether you insist on (definitive) organic feed or not is a matter of religion and a personal choice.
Ex bat feeds which are claimed to be formulated for rehabilitating ex bats and have elevated levels of calcium, protein and a probiotic suppliment are expensive in comparison and I would only think worthwhile for a short period. However remember feed and and suppliment manufacturers market their products to attract owners as chx can't read. It is common practice for smaller suppliers to just formulate feeds for theoretical benefits to give their products a unique marketable feature. They may not have been trialed scientifically against conventional feeds to give actual beneficial results. Of course they sell them at elevated prices too with higher profit margins. The larger manufacturers who supply the commercial poultry industry have to be more scientific and competively priced. You should be able to obtain a good balanced feed at around £6.00 to £7.00 for 20kg. As has been said once chx get used to one feed they need to be weened off this and on to another so if you start them on an expensive feed they will appear to prefer that if you try to make a sudden change. To ween them onto a new one mix the new with the old and increase the proportion of the new over time.
Regards