Trolle,
EU OPS, properly Annex III to Council Regulation (EEC) No. 3922/91 updated by the Annex to Commission Regulation (EC) No. 859/2008 of 20th August 2008 (replacing one inserted by Commission Regulation (EC) No. 8/2008 of 11th December 2007), is applicable under the circumstances prescribed at OPS 1.001. Briefly, EU OPS regulates civil aeroplane operations involved in commercial air transportation and operated by a company whose principal place of business or registered office is in a Member State. There are some exemptions which are similar to those under 14 CFR 119.1(e). Aerial work for example, subject to the condition made under OPS 1.001, is not bound by EU OPS.
JAR-OPS 1 has been replaced by EU OPS.
UK CAP 393 is simply a useful consolidation of current regulations affecting aeronautical activities over which the UK has jurisdiction. Para 1.2 to the foreword states:
1.2 It has been prepared for those concerned with day to day matters relating to Air Navigation who require an up to date version of the Orders and the Regulations mentioned above. It is edited by the Legal Adviser's Department of the Civil Aviation Authority. Courts of Law will however refer only to the Queen's Printer's Edition of Statutory Instruments.
As its contents span several separate instruments the CAP is updated fairly frequently so ensure you check
CAP 393: Air Navigation: The Order and the Regulations | Publications | CAA regularly for the latest edition. The UK Air Navigation Order, Rules of the Air Regulations, and so on, apply (where applicable) regardless of whether you're having fun. Aerial work operations in the UK for example are regulated by the Order. If you are operating in the UK in an EU OPS operation you are still bound by UK law where it applies. The only way for you to have a clear picture is to study the Order carefully.
If you operate a foreign civil aeroplane in UK airspace you shall be bound by the UK Rules of the Air Regulations (and various parts of the Order). If operating a UK registered civil aeroplane overseas you are still bound by the Regulations and the Order. Refer to Rule 2 of the Regulations and Part 32 of the Order. Similar provisions exist under the laws of other states and, although I have not read the German equivalents, it would be expected that Germany requires its civil aeroplanes to be operated in accordance with its own laws together with those in force where the aeroplane is flying, when operating overseas. If a conflict of laws arises one normally follows the laws in force in the airspace being operated in.