Hi Andres,
There's not a short answer, but in summary:
For a temporary visit you could probably get approval for an extended exemption from having a standard CofA.
If you are moving permanently, then you would need to register it in the country you are living and gain a permit to fly in that country. There may be some latitude to register in an adjacent country where you have connections, but that is not guaranteed.
I believe the French are more restrictive on importing amateur built aircraft. It may be worth contacting the Vans Club de France for advice.
In the UK it is accepted, but you would have to demonstrate compliance with the 'Type Acceptance Data Sheet' for the RV-8. Approval can take 2-6 months depending on various factors. The LAA are reticent to recommend permits to G- registered aircraft not based in the UK.
If it is just a single extended visit, I would suggest pursuing the exemption.
You will probably have to get insurance from a European underwriter as not many US providers will cover you to EC requirements.
I'll drop you a PM so that you can contact me directly for more detail.