Your items will show up in main default results when your items match buyers' search terms. However, there are instances in which your items will show up somewhat differently.
Because categories are sometimes different between sites, there are some cases where items won't show up on the secondary site exactly as they do on the primary site.
Your item will always show up on the primary site (i.e. eBay.co.uk), as normal, when a buyer does a search. This will not change.
The only change is that you now have the opportunity to have your item show up in search on the secondary site.