If the controller had required you to go around immediately he/she would have used "G-CD Go around I say again go around. Acknowledge".
Controllers don't issue go-around instructions on a daily basis. I would assume that the controller, particularly since he warned you about the go-around possibility already anyway, simply forgot or deliberately neglected to add "I say again, go-around, acknowledge". But that doesn't really alter the meaning of the instruction.
Also, what the "not below 450 feet" means, probably depends on the context. If your circuit height is 1000 feet and you just started your descent, he may have meant that it would be OK for you to continue your approach to 450 feet, at which point you needed to start the go-around. But if the circuit height would've been lower, or you were already significantly into your descent, it may have been an instruction about the altitude for the subsequent circuit. In other words, go-around immediately, climb to at least 450 feet and fly your circuit at at least that height.
Anyway, you did the right thing by going around. The rest is just semantics and I'm sure that if anything was unclear in the context, you two would've cleared that up right there and then. I've had a go-around instruction a few months ago, which did not include a circuit altitude, and at my home base we can fly the circuit at 500', 1000' and 1500' depending on ATC instructions. So I simply told them I would be climbing to 500' and they agreed. (I knew the circuit at that altitude was free anyway.)