There is no legal or regulatory requirement for you to carry a letter for gender-affirming medication, such as testosterone, when you travel. All you need to do is to make sure your medication is in the original pharmacy packaging with your name on it.
However, if you are concerned and you would like a letter to confirm that this medication was prescribed to you, we can create one for an administrative fee of £25. This can be requested here: https://www.gendergp.com/help-centre/document-services/