I'm playing around with R Rayshade, which creates lovely 3D terrain images using GeiTIFFs. I've been testing it out with SRTM 1 Arc-second images from USGS, but these are quite low resolution. OS provides large scale elevation data in Terrain 50. These come as ESRI shapefiles; when I load them into QGIS they appear as contour lines. How can I convert these into a DTM TIFF? I have tried the TIN interpolation tool but it crashes QGIS hard every time, so I suspect I'm not doing something right.
The data is also available in GML, ASCII Grid & GML, and Geopackage, if any of those are easier to work with.