Are you saying that you are doing this website merger for the sole purpose of using reviews.co.uk for both stores? The reviews site allows you to have multiple subdomains per paid account, so you don't need to merge the sites. All you need to do is configure a subdomain for one of the stores. For example, use www.zero-waste-club.com and trade.zero-waste-club.com. You can then display the reviews on both, without changing the sites or worrying about custom code.