Good news: After lots of hours research, contact with different institutes across the country, quality checks etc. there’s for the first time a high-quality DTM of the whole
United Kingdomhttps://data.opendataportal.at/dataset/dtm-united_kingdomFor about 75% of England, Wales, Northern Ireland fine LIDAR source data available, Scotland
unfortunately (beside river valleys and coasts) is not covered as well. For the areas not covered with fine LIDAR-data I used OS DTM-50m which is wider gridded but also based on LIDAR source data and hence especially in wooden areas more accurate than SRTM data.
Enjoy the new DTMs of the UK!