Nice work so far Rob.
Lee, theis job is going to be a lot easier than you think, as neither of these weapons were 'gun metal'.
First, the Bren. The butt stock and pistol grip were a very dark wood,close grained, so no 'streaking' Paint these dull brown, about the colour of Milk Chocolate, with the reiforcing strap along the butt black. The receiver (the main body of the gun) was black on the Mk2, and a dull dark grey on the Mk1, with the magazines on both starting out black, and ending up as a dull dark grey. The barrel was a mid grey metal colour. Paint this mid grey, and then rub a pencil, or powdered pencil lead over it. The gas tube, below the barrel, was black, with the gas port block (the protrusion beneatht the berrel, near the front, with a rounded bottom edge) was a steel colour. Paint the latter silver, with a touch of matt black in it. The bipod was either black painted, or a dull grey metal - again, a darkish grey with pencil rubbed over it will work.
The SMLE used a darker wood than the later Enfield No4 Rifle. This wood was a similar dark brown, 'grainless' colour as the Bren. All fittings, including the magazine, were a semi-matt black, with the exception of the bolt itself, and the rear tangent sight, which were a dull grey metal colour, reproduced as above. The butt plate was plain brass. Mix this from yellow and dark earth, with a tiny spot of silver. The carrying slings for both weapons were canvass webbing, with brass fittings, the webbing being a dark sand colour after use.
Hope this helps.