Is it legal to purchase testosterone therapy online?

Yes, it's legal to purchase testosterone therapy online in the UK only through legitimate, CQC-registered private clinics that provide a full medical consultation (including blood tests) with a GMC-registered doctor before prescribing; it is illegal to buy without a prescription or from unregulated sources as it's a Schedule 4 controlled drug, and you must have a valid prescription for any purchase, even from online providers.