Membership is FREE, giving all registered users unlimited access to every Acorn Domains feature, resource, and tool! Optional membership upgrades unlock exclusive benefits like profile signatures with links, banner placements, appearances in the weekly newsletter, and much more - customized to your membership level!
In order to register a .de ext you need a German registered address, there are plenty of companies offering that service if you don't have one.
The laws are also a little different, if some one has a company or a name the same as your domain and you do not use it as your company or name then they have the right to claim the domain. In most cases they will win and if they can prove you have a UK address they can also claim the domain no matter their business or name. So don't publicly reveal the domain
As for the best registrar, that has to be down to personal preference, i would use the more established simply because they are more likely to have the experience with .de ext. 123-reg would be a good choice in my opinion
Never done a .de transfer. But domaindiscount24 were the only registrar I found who could properly sort an .eu transfer for me.
There a German company i think and they provide special services for .de registration outside Germany so there a good chance they will be able to sort you out.