You might want to take a look on the MoDELS conference http://www.modelsconference.org/, and the ER conference http://www.nwpu-bioinformatics.com/ER2018/
Both of them are about modelling, althought not restricted to UML, and appear in the category A- of the GGS rating (http://valutazione.unibas.it/gii-grin-scie-rating/ratingSearch.jsf)
An intersting conference so-called International Conference on Model & Data Engineering (MEDI) will be held from 24 to 26 October 2018 in Marrakesh, Morocco. Proceedings will be published in LNCS by Springer Verlag and extended papers will be furthermore published in relevant journals. Please see the web site:
International Conference on Advanced Information Systems Engineering (CAiSE). It is not dedicated to UML mais its focuses among others on information system modeling.