I recently had to look for a new yoga class to join because the teacher whose classes I had attended for the past ten years or so was taking a break from teaching. I was rather disappointed because I thought that it might be difficult to find a good replacement for my old teacher.
It took me a few tries to find a new teacher who is (a) local, (b) good, and (c) runs sufficiently challenging classes, but I've managed to find someone who is great. She is called Elli and runs regular classes in Kennington near Oxford.
Elli teaches hatha yoga. The classes are very well structured and quite demanding -- focussing on increasing core strength as well as maintaining flexibility -- but they are also fun and easygoing. Elli herself is lovely -- very friendly and relaxed. The classes take place in a large school hall, so there is plenty of space.
If you're looking to join a yoga class in or near Oxford, I would recommend this one. You can find out more about Elli's classes on her website.