Having taught innumerable correspondence courses to German managers and staff, including how to differentiate between formal and informal forms of address and closes, I would recommend changing the close to "With kind regards" if you want to express the change in register, which is the next degree of informality in an all-purpose close in English correspondence. The books I used were all of British origin, and a very common formal British English sign-off still in use is "Yours faithfully". As already stated several times, we don't really use the "collegial" part in an English close that I'm aware of.

Hope this helps. :-)