The following link allows you to download a composite 2m DSM/DTM:
Are there any freely available datasets containing DTMs which are in <= 1m resolution, perhaps a similar link in the data.gov.uk site or indeed any other site?
The text on that very page reads:
Data is available at 2m, 1m, 50cm, and 25cm resolution.
and I found 50cm LIDAR Composite DSM & DTM for Scotland just searching for LIDAR Composite Scotland on data.gov.uk
I didn't see the 25cm, but you can do more searching yourself, or contact the responsible agencies directly with a request for the data.
Only for a couple of small areas but the Natural Environment Research Council (NERC) Airborne Research Facility have some LiDAR point cloud data for Scotland which should be sufficient point density to create a 1 m DTM/DSM from. A map of available flights is shown below. These data are freely available to download from http://data.ceda.ac.uk/neodc/arsf/ but you need to register for an account and apply for access to the ARSF archive first.
Disclaimer - I work for the NERC Airborne Research Facility Data Analysis Node.
At the moment, to my knowledge, there is no complete LIDAR DTM or DSM for the UK that is open data. I have been looking for years.
The open LIDAR that is available is open because it was flown by the Environment Agency to carry out flood risk analysis. Therefore the coverage is limited to river catchments, coastal areas and low lying flood plains.
Here is a link to a catalogue of the LIDAR data on data.gov.uk by Chris Guitteridge https://github.com/cgutteridge/uklidar
Here is a good summary, it's a bit dated as it is Oct 2015, but it is a good read. Collecting LIDAR data is expensive so complete coverage is unlikely to be open data anytime soon.