What is the best method calculating travel distance between destination and point layer in QGIS when using OSGB 1936 / British National Grid coordinate reference system?
The question of "what's the best method" is entirely subjective and specific to your situation. The best way to answer this question, in my opinion, is for you to research the different methods and try out the one(s) that seem to best fit your needs. Here's some information to help you get started.
As was discussed in the comments above, calculating straight-line distance ("as the crow flies") is a relatively simple task, and there are many ways to do this in QGIS (eg Distance Matrix).
On the other hand, calculating travel distance is more complicated. This is also called
- "network analysis"
- "shortest path"
- "path finding"
With these search terms you will find many sources that explain the different methods available in QGIS. For example:
- ORS Tools plugin
ORS Tools provides access to most of the functions of openrouteservice.org, based on OpenStreetMap. The tool set includes routing, isochrones and matrix calculations, either interactive in the map canvas or from point files within the processing framework. Extensive attributes are set for output files, incl. duration, length and start/end locations.
- Online Routing Mapper plugin
Generate routes by using online services (Google Directions, Here, MapBox, YourNavigation, OSRM etc.)
- GRASS tool
v.net.distance. This tool requires a road network layer. You can obtain roads from Open Street Map, the British Ordnance Survey or from other sources (see Open Data Stackexchange).