Well, expanding your knowledge and wanting to learn more is a very good thing. However, if you plan to work as a locum for a big multiple chain you should know that they are not willing to pay you more for your extra knowledge. There are several courses to do Therapeutics. If my memory is not bad I believe one good university is the City University. The courses are not cheap and you have to pass some exams. Once you get your title, it will be valid only for 3 years because you will have to do the necessary CET points to keep it running. Otherwise it will expire. The Therapeutic CET points required will be extra on top of your normal ones for the same period.
In my opinion if you plan to work in a hospital or in a practice where therapeutics is used (eg there are some practices registered for quick eye emergencies depending on the PCT), then it might be very useful. The Optom who is allowed to work for one of those local schemes has to be registered with the local authority. This option will be better for full time optoms working in the same practice.
Iam not trying to discourage you. However before you spend a lot of money, time and effort studying a therapeutics course you should consider if it will have a good return (economic or professional) to you.
I hope my views might be helpful.