Is there any guidelines in BS EN using the ratio of concrete 1:1.5:3? If there is any, what class of concrete they are?

Yes in India, the guidelines are expressed under following codes. a)IS 10262 (2009): Guidelines for concrete mix design proportioning b) IS 456 (2000): Plain and Reinforced Concrete – Code of Practice

Volumetric design is obsolete these days. Used only for small and trivial designs !!

For M20 nominal mix concrete the mentioned mix is used, For reference only you can refer to IS 456, page number 23, link: http://civilengineer.co.in/wp-content/uploads/2015/02/IS-456-2000-Plain-Reinf-Concrete.pdf

You can take guidelines from BS8500 for it. Class C 20/25 here applies with Portland Cement 325 kgs, along with admixture to get better strength.