Because only 'supernumerary' research (i.e. activities which do not form any part of ongoing research within the host department/faculty/college, and would not be carried out if the visa holder did not come to the UK) is permitted under the Tier 5 Sponsored Researcher route which the University sponsors, Tier 5 visa holders are not permitted to undertake teaching as part of the purpose of their visit.
A Tier 5 visa holder in a research role can give lectures and talks on their research, as long as this does not form any part of a taught course. If giving lectures and talks has been mentioned as part of their activities in their Tier 5 Certificate of Sponsorship (CoS), a Tier 5 visa holder would then be permitted to undertake research and/or teaching as supplementary employment.
A Tier 5 visa holder whose CoS does not make any mention of lectures or talks would only be permitted to undertake research as supplementary employment. If they wished to undertake teaching, they would have to ask their host department/faculty/college to submit a Changes & Leavers form to SIT to report that they would be giving lectures and talks as part of their research activities. They would also have to submit a letter from their funding organisation confirming that they have no objections to their undertaking these activities. SIT would then have to report the new activities to the Home Office before any lectures or teaching commenced.
It is not possible to add teaching duties if the Tier 5 visa holder's funding organisation does not confirm their consent.