Measuring tool and field calculator giving wrong area in QGIS 3.10.4

I’ve been consistently having issues with the QGIS measuring tool in 3.10.4. When I measure an area or line (regardless of the units) it gives a wrong value when compared to the measuring tool in Google Earth. This also applies when using the $area in field calculator. I am guessing this is a CRS issue, possibly related to the base map. I am using EPSG 27700 (British National Grid) and all my layers are also set to this.

The base map I am using is a Google aerial from QuickMapServices plugin, which I have manually set to convert to EPSG 27700 as otherwise it shows up somewhere completely else. If anyone has any ideas that would be really helpful, as I’ve tried to fix myself to no avail.