Just upgraded to QGIS 3.8 and opening a brand new empty project, changing the CRS to epgs:27700, gives an error
The preferred transform between EPSG:27700 [OSGB 1936 / British National Grid] and EPSG:4326 [WGS 84] is not available for use on the system. This transformation requires the grid file “OSTN15_NTv2_OSGBtoETRS.gsb”, which is not available for use on the system. This grid is part of the proj-datumgrid-europe package, available for download from https://download.osgeo.org/proj/proj-datumgrid-europe-latest.zip. Current transform “Inverse of British National Grid + OSGB 1936 to WGS 84 (6)” has an accuracy of 2 meters, while the preferred transformation “Inverse of British National Grid + OSGB 1936 to WGS 84 (9)” has accuracy 1 meters.
Notwithstanding the fact that the grid file presumably shouldn't be missing, even the current transform (which claims to have an accuracy of 2m) places any 27700 layer in the sea just off South Africa.
The exact same process with 3.6 (on a Manjaro machine not yet upgraded) does not yield this error.
I've raised the issue at QGIS git, but it doesn't seem to be replicable on other systems, so possibly just a Manjaro/Linux version issue.
I just wanted to check in with anyone who uses QGIS (@dobedobedo being the only one I can determine from threads here) . If you've time, could you please open a new blank project, go to Project>Properties>CRS and select 27700, and let me know if the error turns up. Better still, if you have one, load a 27700 layer into a project whose CRS is not 27700 and let me know what happens.