I'd always understood that, because there is no en-EU IETF tag, to target Europe I'd have to either use en or list out an hreflang for each country within Europe.
Looking at the source of Think With Google, I noticed the following:
<link rel="alternate" href="//thinkwithgoogle.com/intl/es-419/" hreflang="es-419" />
Which appears to use a UN M.49 code to indicate Spanish for the Latin America and Caribbean region.
The UN M.49 Wikipedia page also lists a code for Europe - 150.
Does it follow that I could have the following hreflang tag to indicate English for the European market?
<link rel="alternate" href="//example.com/intl/en-150/" hreflang="en-150" />
If you are not going to specify languages for each European country just set hreflang="en". If you are going to specify English for each European country be aware that only IE (Ireland), MT (Malta) and GB (Great Britain) have English as their official language.