A couple of thoughts here.

Are you going to take over LMOP? Are you going to run another module? Running a homebrew campaign? This might affect everything. I'm assuming you're going to be running something other than LMOP, but hey, I could be wrong.

Anyway, on to my point.

If the campaign started while the UA materials were out, it would have been best to specify whether or not they were off limits.

The campaign my group is currently running started before any of the supplements were out, so as each supplement came out, I talked to them about it before we were in-game and we discussed what we thought of using it.

However, I think the entire group should talk about the use of the UA supplements and what everyone thinks about using them. There could be a lot of reasons why the DM might not want to include the supplements, if for no other reason than that they're not fully official yet. That being said, I think this is something you all should talk about as friends (not as players vs. DM) and not right before/after or during game time. Since you all are going to be playing together, you should definitely figure out opinions and thoughts about all this before you get past the "intro" and into the "full" campaign.

There's a difference between the DM envisioning running the LMOP campaign without using the UA supplements and just saying "no supplements because I said so." However, a good DM should take their players opinions into consideration.

To kind of flip this around, what if, while you were DMing, you let everyone know the UA material was fair game and they all told you they didn't agree and didn't think the supplements should be used? (I know, highly unlikely, but still an interesting thought.)

Since you're going to be taking over the campaign in a few weeks, I'd say whatever happens regarding UA currently doesn't REALLY matter that much. Be considerate of one another and everything will be fine. Once you start DMing, just make sure to let everyone know UA is fair game.