Tax treaties are negotiated by HM Revenue and Customs (HMRC) officials, reporting to Treasury Ministers.
The UK has a large number of tax treaties which includes treaties with developing countries. Some of those treaties require updating and HMRC officials actively seek engagement with developing countries to that end. For example, there are ongoing negotiations with Ghana, prospective negotiations with Nepal and negotiations with Lesotho and Malawi are close to conclusion.
HMRC publishes the programme of tax treaty negotiations and news of signed treaties at: https://www.gov.uk/government/publications/double-taxation-agreements-developments-and-planned-negotiations.