houseprices.io experimental area
The composite data covers over 72% of England (no Wales, Scotland or Northern Ireland yet as this data's held by the devolved agencies). Resolutions available are 25cm, 50cm, 1m, and 2m, but coveraage appears very limited at the higher resolutions. The 1m data appears to strike a good balance of coverage and detail, so have mostly been looking at that.
Have grabbed some of the DSM 1 meter data and drawn some map tiles with contour shading shading to produce a zoomable map.
Data covering an area not quite the size of Wales has now been released by Natural Resources Wales. Sadly it appears coverage at 1m resolution is a lot thinner than in the Environment Agency data. On the other hand, unlike the Environment Agency, Natural Resources Wales have released pre-rendered images available to download as OS grid geocoded JPGs or consumable as map tile PNGs via API endpoints. They've also provided a slippy map 'preview' tool which includes full detail DSM and DTM tiles at all resolutions, including at 2m for which coverage is much more comprehensive.
We've added the DSM 1m data to our map anyway.