Question to the Home Office:

To ask Her Majesty's Government how many visas they have refused to delegates wishing to enter the UK to attend international scientific conferences; how many of those refusals were for applicants from low and middle income countries; and what were their reasons for such refusals.

The information requested is not available in published data. Visa nationals travelling to the UK for this purpose would normally apply for a visit visa.
The large majority of entry clearance visit visa applications are granted. Information on grants and refusal rates for entry clearance visitor visas is published in the quarterly Immigration Statistics, Visas volume 1, table vi_01_q, latest edition at:
https://www.gov.uk/government/publications/immigration-statistics-year-ending-september-2018/list-of-tables#visas