I can only answer about marriage in Germany. One condition is that one of the two is a resident of the area of the Standesamt (registration office) where the marriage is conducted, so you would have to get married where she lives in Germany (a man living at town A and a woman living in town B could choose to get married in A or B, but not elsewhere). The other main condition is that the marriage must meet all legal requirements according to German laws and according to the laws where both are citizens, that is UK law and your country's law.
Your wife to be can contact the Standesamt where she lives, and they can obviously tell her the legal requirements according to German law, and requirements according to UK law, and most likely (unless you come from a very obscure country) they will know the legal requirements of your home country. If things need to be done in your home country, a consulate or embassy will count as your "home country". For example, she will have to visit a UK consulate in Germany because the wedding has to be publicly announced in the UK for 14 days. The Standesamt will also know what you need to do to make the marriage recognised in your home country, and what consequences there will be on citizenship.
Any papers you need to provide will have to be translated to German by a qualified translator.