I remember reading something like what you stated some years ago, but checking now the official UK GOV site it clearly states that it does not work like that anymore:
When entering the UK, you need to prove your medicine is prescribed to you if it contains a controlled drug - find out what proof you need to get.
www.gov.uk
And even testosterone is on the list of controlled drugs:
www.gov.uk
So for Testosterone, schedule 4 Part2 drug you need a proof letter:
"If it contains a drug listed as schedule 4 (part 2)
To make sure your medicine does not get taken away from you at the border, carry a letter of proof that the medicine was prescribed to you."
The only thing I have been able to find regarding the 3months supply, etc is checking the Personal licences:
Information on carrying medicine containing controlled drugs for individual travellers entering or leaving the UK.
www.gov.uk
"Personal licences
The personal import policy exists to enable prescribed controlled-drug containing medicines to be carried on your person when you visit the UK, without a licence being issued to you.
When you do not need a licence
You do not need a personal licence if you are travelling with:
- medicine listed in schedule 5 of the regulations
- medicine listed in schedule 4 (part II) of the regulations
- less than 3 months’ supply or travelling for less than 3 months with any schedule 2 to 4 (part I) drugs which have been lawfully prescribed to you in your country of habitual residence."
So Test ( 4 part2) does not need a personal licence, but still need to meet the requirement of proof letter.