A lot of people in the UK expect to learn tai chi for free. In fact, there are a number of teachers who offer free tuition.
It may be worth looking closely at what is on offer:
Are they offering tai chi chuan (martial art), tai chi for health, tai chi-style exercise or qigong?
Is it tai chi for the over 50's?
When did the teacher start practicing tai chi?
How long has the teacher been offering lessons?
Who did they learn the art from?
Is the school teaching a recognised style of tai chi?
Do they offer the 8 areas of study?
Are you discovering the essence of the art?
Free does not necessarily mean authentic.
When something has value, people seldom want to give it away for free.