Dear Ivaylo
Do you mean the 1935 (not 1931) International Folk Dance Festival held in London and organised by the English Folk Dance & Song Society (EFDSS) and Maud Karpeles? Raina Katsarova attended and gave a paper on Lazarnica which was published in the festival issue of the Journal of the EFDSS, 1935, which can be accessed through JSTOR. The dance group attending the festival was Bulgarska Kitka, led by Boris Zoneff.
best wishes
Derek Schofield