Then in that case I would look elsewhere unless you feel comfortable enough in the dojo that you think you'll be happy there for 12 months.
Legal action seems a bit harsh, but you also said that you've had until now to 'sample' the dojo, so it's perhaps a fair-enough proposal. The McDojos that [literally] knock on my door won't even let you watch a class before you hand over hundreds of pounds to them.
If your sensei never approached you with a contract, do you think you would have continued training there for several months to come? For me, that would indicate my answer.
Yes. Because if I decided I didn't like it or there were things that popped up that I couldn't live with, I could leave. If I had joined, I would have had to stay or pay as if I were there. And it would seem there might be other hidden costs or obligations that I wouldn't know to ask about, even if I could ask, before signing the contract.