As you've discovered, the plainnat bibliography style does not abbreviate given or "first" names. To change this behavior, as well as to have the given name(s) placed after the surname, I suggest you perform the following steps:
Find the file plainnat.bst in your TeX distribution and copy it to, say myplainnat.bst. (Don't edit a file provided as part of your TeX distribution directly.)
Open the file myplainnat.bst in your favorite text editor and search for function named format.names. (It starts on line 216 in my copy of plainnat.bst.)
In this function, look for the line
{ s nameptr "{ff~}{vv~}{ll}{, jj}" format.name$ 't :=
Change this line to
{ s nameptr "{vv~}{ll}{, f.}{, jj}" format.name$ 't :=
Save the file myplainnat.bst either to the directory that contains your main .tex file or to a directory that's searched by your TeX (and BibTeX) distribution. (If you choose the second option, you may also need to refresh the filename database of your TeX distribution suitably.)
Start using the modified bibliography style file by issuing the command \bibliographystyle{myplainnat}.
Incidentally, since the plainnat and, by extension, the myplainnat bibliography styles use [English] "sentence style" to typeset the contents of a title field in an entry of type @article, you should encase words such as EU, Referendum, Finland and Maastricht in curly braces to keep them from being typeset in all-lowercase.
Addendum: It's been suggested that I provide a bit more information about what the strings "{ff~}{vv~}{ll}{, jj}" and "{vv~}{ll}{, f.}{, jj}" in the format.names function stand for.
Some background information first. As far as BibTeX is concerned, an author's full name can have up to four separate components: A first name component (which includes any middle names as well), a "von" component (e.g., "de", "von", "van", "van der", "della", etc.), a last name or surname component -- usually, but not necessarily, a single word; a rather spectacular deviation from this rule is the four-word surname "Mies van der Rohe" -- and a "junior" component (e.g., "Jr.", "Sr.", "III", etc). A full name must have at least a "last name" or "surname" component; the first-name, von, and junior components are optional.
As you can probably guess by now, the strings ff, vv, ll and jj above denote the four components; the form "ff" instructs BibTeX to print the complete first and middle names (if present, of course), whereas the form "f." instructs BibTeX to print only the first initial of every first and middle name.
There are various syntax rules that determine how BibTeX parses a full name to arrive at the various components. While this isn't the place for a full-blown tutorial of BibTeX syntax, it's worth mentioning a couple of topics:
BibTeX does not examine the interior of the first-name component for special characters. Suppose, say, that an author's full name is listed as "C.A. Brown" in your .bib file. BibTeX will correctly identify "C.A." as the first-name component, but it will not ascribe any special meaning to the "." (dot) inside "C.A.", i.e., it won't notice that the field's first-name component consists of two separate initials, each one terminated by a dot. Thus, if the "{ f.}" parsing specifier is provided in the .bst file, BibTeX will abbreviate "C.A." down to "C.", even though that's probably not what you intended it to do.
So, what should you do if you do want to show all initials of the first-name component and not just the very first initial? You need to write the author field as "C. A. Brown" or, equivalently, as "Brown, C. A.". That way, BibTeX will "notice" that the first-name component has two parts ("C." and "A.") and will dutifully render both.
As noted above, BibTeX assumes that the name of every author has at least a last-name component -- and possibly as many as four components. This factoid is useful for treating entries that have so-called "corporate authors", e.g., "National Aeronautics and Space Administration" and "International Monetary Fund". Left to its own devices, BibTeX will interpret the former author field as having two [!] (recall that "and" is a reserved word in author fields) separate authors, the first one named "National Aeronautics" and the second one named "Space Administration"; for the latter author field, its syntax rules will lead it to conclude that there's a single author with first name "International", middle name "Monetary", and last name "Fund".
If the bibliography style specifies that first names should be abbreviated and listed after the respective surnames, BibTeX will end up typesetting the author fields as "Aeronautics, N. and Administration, S." and "Fund, I. M.". In addition, the former entry will be sorted under "A" (for Aeronautics) instead of "N", and the latter entry will be sorted under "F" (for Fund) instead of "I". Moreover, if author-year citations are being generated, they will look like "Aeronautics and Administration ([year])" and "Fund ([year])". Clearly none of these outcomes can be what you had in mind!
The solution to the corporate-author problem is simple: Encase the entire author field in double curly braces. For the two examples above, one should write
author = {{National Aeronautics and Space Administration}},
and
author = {{International Monetary Fund}},
respectively. The outer pair of curly braces delimits the field, and the inner pair serves to make BibTeX treat the field as having only one component -- which, by its syntax rules, must be the last-name component. In a sense, then, BibTeX is made to believe that the former author has one complicated last name consisting of five words and the latter author has a last name that consists of three words. The happy result, though, is that the names will be rendered -- and sorted -- correctly.
filecontentspackage to 'bundle' the.bibfile with the.texsource. Not relevant to the question, but you shouldn't haveJahn, Detlef; Storsved,Ann-sofiefor the author list: you always needand, soJahn, Detlef and Storsved, Ann-Sofiewould be correct.plainnatstyle? If so, there are at least a couple of approaches which come to mind.