FHRS data is a rich source of open postcode and address data that can be used in OSM. I've developed a web application called FHODOT (food hygiene open data OpenStreetMap tool) to visualise matches between FHRS and OSM data and efficiently add fhrs:id, addr:* and addr:postcode tags to OSM objects through JOSM or iD. For more information, see the FHODOT readme.
I've produced a GPX file which contains the OS triangulation station archive filtered to inculde only those stations not marked as destroyed. Co-ordinates have been transformed to ETRS89 ("a more exacting definition of the GPS co-ordinate system than WGS84") using the OSTN02 transformation and the file includes the name, description and type for each point in a format that JOSM can understand and display. (You may wish to go into Edit > Preferences, make sure Expert Mode is on and change Waypoint labelling to {name} ({desc}) - {type} in order to show the type for each station.
It is my hope that the OS trig archive may be imported into OSM at some point, but in the meantime this GPX file could be used to accurately determine the offset of aerial imagery.
I've written a BASH script which uses GDAL to automatically process Environment Agency LIDAR 'composite' data and create map tiles which can be viewed in a browser or used in JOSM. Map tiles are coloured based on the height of surface features such as buildings and trees, calculated by subtracting the DTM from the DSM. This can make it easier to trace buildings more accurately because different roof heights are different colours. By default, features below 1m are transparent, meaning that buildings can easily be distinguished from ground level and the tiles can be viewed on top of Bing imagery, for example.
I've created map tiles for the area around Canterbury, which can be added as a TMS imagery layer in JOSM using this URL: http://gregrs.dev.openstreetmap.org/lidar/{zoom}/{x}/{-y}.png
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Fingerprint: EBD1 077F CCDD 841E A505 3FAA D2E8 592E 6490 7C8A