There is no need to obtain a UK transit visa if you already hold a more senior visa of any type. The primary legislation for this situation is in Section 33 of the Immigration Act 1971, which is referenced in Appendix V of the current rules...
visit visa means an entry clearance for the purpose of a visit under
section 33 of the Immigration Act 1971. It is normally a vignette in
the holder’s passport. It includes entry clearances for visitors that
were issued under paragraphs 40-56, 56D-56J, 56N-56Z, 75A-75M of these
Rules and Appendix V
Based upon what you wrote, you have a visitor visa which will be in force during your transit. By default, these visas are multiple entry with a term of 6 months. During that period you can engage in any of the "permitted activities", which includes transit.
Related question: Do I need a transit visa if I already have a UK Business visit visa
See also: Do I need a UK airside transit visa, if I already have a UK visitor visa?