Detector vans are mostly a hoax. It's thought that they worked by detecting electromagnetic radiation given off by CRTs, if they worked at all. Those are obsolete, so the point is moot anyway. But how should they be able to tell apart a TV and a computer monitor? Furthermore, you can use a TV and not connect it to an antenna—Smart TVs and Chromecast/Roku/Fire TV/Apple TV/... make that a perfectly viable use case. Such use isn't subject to TV licencing in the UK, only watching live broadcasts (including streaming) is.