Hi Alan and Matt,
I am sorry to tell you that if you set up the hreflang for "Europe" as hreflang="en-GB", that won't work.
That annotation, in fact, tells Google to show the URLs having it only to users searching in English from Great Britain.
It would be better to use only "en" in European website.
Said that, this is not the best solution either, because it is telling Google: "show this to users searching in English globally (but not if they are in the USA).
If the European web site is meant to reach users who not necessarily are using English as default language (eg: Spanish, French, Italians, Germans et al), than a solution could be tagging the European website with the "x-default" hreflang.
Note, though, that this a quite extreme use of the x-default.
The big mistake, anyway, is creating an European website itself:
- Google does not consider political regions like European Community nor continents and geographical areas like "Asia", "Middle East", "Europe";
- Because of 1, you cannot geotarget a website but for Political States (Spain, UK, Russia...)
- To think that not-English speaking users will use English for searching something it is not realistic, therefore it is correct what Matt says in his answer re: translated versions served in whatever format (ccTld, sudomain, subfolder) better fits your business needs.
Finally, personally I would not suggest to a ccTld for targeting European users, because that ccTld would geotarget the site to its country (eg: .es to Spain). Better a generic domain name (.net or even .eu, which is a generic domain name and does not have any geotargeting power), or even a subfolder/subdomain.
Finally, when creating the different country sites, I remind you that in certain countries is spoken the same language. For instance Ireland and UK share English, but they have different currencies, obviously different postal system and phone numbers and, especially, a different culture, so that you should not think in having an European EN version serving all the English speaking countries, but localizing each one of them.
To not talk, and I really end my answer, countries like Switzerland (French, German, Italian and Romance), Spain (Spanish and Catalan), Belgium (French and Flemish), Ukraine (Russian and Ukranian).