There are plenty of experts within the FBA field, you can Google them or seek help here on Clarity.
Before you seek help, perhaps you want to look into these marketplaces and check for entry requirements and market research.
For instance, you need to register for VAT to sell within the UK. Check out the UK government site for more details: https://www.gov.uk/vat-rates
You can seek help from accountants over at ukvat.uk.com instead of the big 4 as bigger firms will take longer time to get back to you.
As for EU marketplace is simpler once you have applied and started selling within the UK marketplace. Germany now requires sellers to have VAT as well.
Selling within the Japan marketplace is pretty straightforward.
Keep in mind you will need to translate your product listings when selling to non-english countries. You can easily hire a translator on Upwork.com and make sure the freelancer is fluent in the native language to avoid any direct translation mistakes.
Every country and marketplace that you choose will have different rules and regulations so be sure to check them carefully.
Stay laser focus on the marketplace you want to expand to instead of trying to sell within all of them. Get familiar with one and generate sales before moving onto the next.
If you need more help selling within the UK marketplace, I am happy to help out as I have clients selling within US and UK marketplaces.