I am not sure this answers your question but here it goes:
If you travel into the sea at night you will be able to find the most beautifull sky. From my experience, being around 15 miles offcoast should already give you a very nice sky panorama.
Another option is finding a dark sky reserve. It's just like a natural reserve but applied to the sky. Some criteria has to be met for this. In Portugal, Alentejo you can find such a dark sky reserve. It was the first spot on earth receiving the “Starlight Tourism Destination Certification” given by the starlight foundations and supported by Unesco. More information in portuguese can be found here. Meanwhile I found the official? website here. They provide an english version.
It's intersting to check the criteria to have such certification:
- Clouds: > 50% nights withoug clouds (Dark Sky Alqueva Reserve has more than 286 nights
- without clouds)
- Darkness: > 21mag/arcsec² (Dark Sky Alqueva Reserve has 21,60 mag/arcsec²)
- Vision: ≤ 2"
- Transparency: mag 6
A quick google search also showed that England recently created a similar reserve in Northumberland county.