Is it possible to use Web Map Tile Service layers in ArcGIS Collector?

Yes. It is possible to use Web Map Tile Service (WMTS) layers in ArcGIS Collector. The WMTS layer must be added as a layer in a web map or used as a basemap in ArcGIS Online. To do so, refer to the instructions provided in ArcGIS Online: OGC WMTS web service.

The image below shows a WMTS layer from the OS Data Hub API added to a web map in ArcGIS Online.

The image below shows a WMTS layer loaded using the OS Data Hub API in ArcGIS Collector.