Take a look at the BECM_SID manual. This thread (https://www.rangerovers.net/posts/416754/) has the download link. Starting at line 21, the programming of the Land Rover is described, including the order in which parameters are set.
Make sure all the other settings, such Engine Type, are right before you execute the Market Set, in my opinion. Since you aren't modifying the BECM's Lockset Code already, you should be able to quickly go into the settings and choose the market.
Get in touch with Les by dialling the number provided on the Key Centre website. He should be able to confirm establishing the market without impacting the FOB match and immobiliser since he conducts BECM fixes.