Think I'll subscribe to this thread! In the process of going through all of this myself so keen to get other people's comments who have already done it.
From what I've read, if you register for VAT (whether compulsory or voluntarily), you have to collect and pay VAT at the rate set in the country you are registered - just like when you buy stuff from ShareIt (Germany) and pay 19% VAT. I think the VAT threshold is £70,000 now. VAT registration if your annual turnover is below that, is voluntary. Otherwise, it's mandatory.