Just a customer… but was I in the wrong?

So, I popped into my local big TESCO, not too busy and just grabbing a box of beer. I get to the checkout, and while my beer is on the conveyor belt, the customer ahead is still packing, and there’s someone behind me loading up their stuff.

Then, a young (I assume) floor leader comes up and says, “You can move to the self-checkout, please.” Since I’m literally about to be served in like 10 seconds, I just say, “Erm, no, I’m okay here.”

She then storms off to a group of young staff, where they start pointing at me and chatting and I didn't feel comfortable with that. Now, I get that it’s probably quicker if I use self-checkout, but I actually like the small interaction with the cashier. So, was I being unreasonable here, or was that a bit out of line on their part?

What is your thoughts here? (good or bad).