Does storefront UK works for whole Europe? My app is aiming at clients living in France, Belgium, Netherlands an UK.

No UK is just UK, and those are all separate stores. You can select 'All' (global), or pick just the ones you want.

It's up to you which countries you sell your app into via iTunes Connect/My Apps/<my app name>/Pricing and Availability/Availability, keeping in mind that not all types of apps are allowed to be sold in all countries.