You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
The comprehensive agreement (SI1993/1802) has effect
In the United Kingdom for
Income and Capital Gains Tax from 1994-95 Corporation Tax from 1 April 1994.
In Uganda for taxable years beginning on or after 1 January 1994.
If using the HMRC Intranet, the agreement can be viewed through the “New treaties/protocols in force” link on the sidebar. On the HMRC web-site, searching for “Treaties in force Uganda” will provide a link to the agreement.
The text of the Tax Treaty can be found via https://www.gov.uk/government/publications/uganda-tax-treaties