Just to add another option:
My guess (though no more confident than @p.s.w.g's answer) is Balanophis ceylonensis (Sri Lankan Keelback) or related species.
The two features that stand out to me from your photo are
- the dotted markings on the back of the snake, and
- the dark line running behind the snake's eye.
You'll notice the keelback in this picture copied from Fernandoa et al. (2015) has an almost identical skin tone and line running behind the eye.
However, this image does not show the dots.
Though a mature snake is very different colored, you'll notice from this picture that the mature keelback has a very similar spotting design as the snake you've photographed.
IF this is the case, and this is in fact your species, then there is at least one source (Fernandoa et al. 2015) that suggests that the snake is venomous.