I am using Ordnance Survey data - Open Data:
OS Street View®
Data type: Raster
Supply format: TIFF-LZW
Selecting National Grid Reference squares
Using the map on the right, or a full size version, identify which square(s) you want to download. Select or deselect squares by clicking on the map on the right.
To manually select or deselect multiple squares from the two letter reference list, press and hold the CTRL key (Windows) or the Command key (OS X) while selecting from the list.
OS Street View is 1:10 000 scale street-level, colour, digital raster mapping that has been specifically designed to cartographically emphasise road carriageways, road names and their Department for Transport (DfT) numbers. Major public buildings are also highlighted and annotated.
Once you download data from OS and unzip it, there will be 2 folders, data and doc. Inside of the data folders, there is a folder containing the TIF data files, and another containing georeferencing information.
The georeferencing files in the other folder contain 2 different types of georeferences, the TFW file (Tiff World file), which is what Cartographica is going to use to position the data on the world stage, and a Map Info TAB file, which is really only used by MapInfo at this point.
Cartographica doesn't support using the TAB file for georeferencing, but considering that the TFW files should serve the same purpose, it shouldn't be a problem.
So, given the organization of the files from OrdnanceSurvey, you will need to place the .tfw files and the .tif files in the same folder. Otherwise, just loading up the tif files without the accompanying .tfw files, the layers will "pile up" on top of themselves.
Once you have copied the .tfw files to the directory containing the .tif files, Cartographica will automatically find them when it loads the .tif files, so you'll be mostly there. The remaining component is setting the CRS. You will note that when you bring the layers in to Cartographica, there will be a yellow triangle next to each layer, this signifies that the CRS isn't indicated in the loaded file, so you'll need to set it to make sure any other data sources line up correctly.
To set the CRS, select all of the layers (click in the layer stack and Select All). Next choose Layer > Set Layers' Projections... which will bring up the projection window. Now, you'll need to find OSGB 1936 / British National Grid either by typing in the search area in the upper right or by opening the Other collection. Once you've selected it, click Set and it'll be set for all layers (and since all layers are the same, it'll set the Map's CRS as well).
Then you'll be set for integration with other data, including live maps.