How to find out about theatre workshops
Theater workshops are happening in pretty much every major city every day and are occasionally free of charge! You just need to know where to look! Most major theaters in areas will offer workshops with visiting theater company's, actors, directors or writers so checking their websites and signing up to their mailing lists is always a good start, as places normally fill up quick. Local Drama schools, universities and some colleges with they're own performance spaces often open they're doors to the public if they have a visiting practitioner, but you can expect to be surrounded by a lot of students studying in that field as that is they're target audience.
I know drama schools like Royal Welsh School of Music and Drama, Bristol Old Vic, and RADA all offer exciting, and high quality workshops in pretty much everything from screen acting to audition preparation, and can range from being a one day workshop to a week, but of course you have to be willing to invest the time and money into them as they are intense and meant as top ups for professionals who feel the need for a quick refresh.
You will also find that following certain artists will lead you to workshops with them themselves, performers like Hadley Fraser and Bonnie Langford have been known to do this in and around London.
Using social networking sites to follow them and the organizations I have already mentioned is a great way to keep up to date with any events that might be taking place, just remember to act fast when they do start advertising, whether they are free or not they are always snapped up fast.
My final suggestion is doing a good old bit of reconnaissance work! Directors, and Choreographers often lead workshops in their spare time as an extra source of income, so taking the time to find out who was involved in the last good bit of theater you've seen is always a good idea, along with finding out where they're bases are so you know which ones are closer to you if an opportunity should arrive.