Which standard dictates how reference designations should be formed in the EU?
Reference designations for electrical and electronics parts and equipment, the Unit Numbering Method, has been around since 1949 when the Institute of Radio Engineers (IRE), predecessor of the IEEE, first came out with it. IEEE 200-1975 has been replaced by ASME Y14.44-2008. Apparently the IEEE figured they had bigger fish to fry and sold the standard to ASME. The US Department of Defense uses it, as does Agilent and some other companies, and, I believe, all NATO countries forces would use it.

A basic reference designator has 1, 2, or 3 class designation letters (if 3 letters the lst letter is X) followed by a (serial) number. See IEEE 315-1975, Clause 22.4 for the "official" list. A 7805 voltage regulator would use class letters VR not U (see IEEE 315-1975, Clause 22.2.4 for an explanation). The A2A1 of A2A1R1 is the reference designator prefix and is explained in ASME Y14.44-2008.

Which standards you choose to follow is up to you but may be dictated by superiors, who may or may not understand.