Brewing British bitters from NB . Says it is a low grav beer. I noted that it had an OG of 1.040. Will it have less alcohol content?
Yep. With a 1.040 gravity you should be around 4% abv.
Low gravity means that it has a low starting gravity. So yes lower alcohol.
(SG-FG)*131 = ABV
SG = OG = Starting Original Gravity
With a lower starting gravity there is only so much sugar in the wort for the yeast to eat. So less potential gravity.
(1.040-1.010)*131 = 3.93% ABV
(1.080-1.010)*131 = 9.17%