If you don’t have a licence, you’ll be flagged up with the licensing people. They operate on a database of addresses, “TV detector” vans are a myth.
You’ll be sent a series of stern letters warning you that your soul will burn in hell, or something. They’ll probably send a man round in due course.
Now. You have two options. You can go “sure, come in, have a look,” and let them go about their business. However, you have no legal obligation to do this, and they have no more powers of entry than I do. So you’re well within your rights to tell them to jog on and come back with a court order, should you wish. For them to get a court order they’d have to have reasonable suspicion that you’re breaking the law, which they don’t, so the chances of them actually getting a court order are slim to none. (And if they do, you can let them in then, you’re not in bother.)
Ordinarily I’d go with the former. However, if they’re going to be shitty about it, I’d be more inclined to be bloody awkward right back at them.
For the record, I have a licence, and always have. I watch broadcast TV and when I see things like the giant cities thing yesterday I think it’s money well spent.