Try to train with a West End entertainment law firm, or try to get an in-house job with a fashion company. Network. You will need to be up to speed on copyright, trademarks, and passing off, and also contract law, agency law, and possibly consumer law. Have fun!
I remember doing a hilarious case for a dodgy Italian retail brand. My client was trying to get the trial adjourned because, he claimed, he was at the shows in Milan. My opponent was hissing sotto voce "He's in Palermo".
Before Universal Thermosensors and Hibben [1992] 1 WLR 850, rag trade Anton Piller orders were a hoot. Turn up at some lock up at six AM, open the shutters and grab all the knock-off Chanel gear. Bish bash bosh, no Supervising Solicitor or any of that malarkey. Things have changed.