For what it's worth my guess would be that this is an early war picture of trainees on a route march or, conceivably a pre war picture of TF troops on a route march at a summer camp in the summer of 1914 - of which large numbers exist.
The picture I use on my website is one example
(bottom picture of a coy. resting on a march)
I agree with SS that troops actually deploying would have had ammunition pouches on.
The caption "Volunteers ready to be shipped to Belgium" does not really make much sense in itself does it? If the idea is that these were WARTIME volunteers (ie people who had signed up after the outbreak of war) - I should think the number of men who volunteered in 1914 with no previous service who made it to F&F in 1914 would be small. Of course at that point (pre conscription) I suppose "volunteer" applied to the entire British Army - but the sense of the caption / picture could be (tough to tell without the full context) "Volunteers [completing their training in preparation for being] ready to be shipped to Belgium" as opposed to "This is a picture of troops about to board a train/boat to Belgium"
My sense from the picture is that it is the former.
One additional piece of circumstantial evidence for this, in addition to the absence of ammunition pouches, is the presence of quite a large number of civillians (or at least men in civillian dress) I count perhaps eight?
More likely to be present in a break in a training march through local towns than in the actual execution of a deployment overseas I would have thought.
just my 2p