I mean, aren’t they all? Or, at least, shouldn’t they be? Otherwise, they’ll sit for seven years in your stomach, like gum.
That is exactly the point, they are high fiber biscuits. One of the Brits brought some to the office when I was in Germany and everyone had tired of the giant keg of Gummi Bears that had been purchased for office snacks.
The word is digestive, not digestable. Of course all biscuits are digestable. The point of digestives is that they are easily digested, as opposed to hard tack for example, which is anything but. The idea being that they are made of materials that are either readily absorbed by the gut or else pass straight through. They don’t hang around for hours causing indigestion.
So, are these biscuits supposed to help you digest (and expel) other things you eat? Or is the point just that the biscuits themselves are easily digestible?
“Digestive” makes me think more of something related to the physiology and biochemistry of digestion, e.g. “digestive enzyme”, or “digestive tract”. I figured “digestive” was an ideosyncratic way, as related to biscuits, of describing them as “digestable”.
It would appear that “digestive” is meant to mean “amenable to digestion”. That could imply a) the biscuit can be digested easily, or b) the biscuit facilitates digestion, either of itself, or other things being consumed.
What this may (or may not) say about any potential laxative properties is unclear to me.
As this 1913 dictionary states, a digestive may be anything which aids digestion.
This page from United Biscuits (owner of brands such as McVities and Jaffa Cakes) says
Baking soda aids food digestion? 
Hrm. It’s a good antacid, which would help with “indigestion”. I usually take that to mean “dyspepsia” (bellyache), rather than literally an inability to digest. I suppose indigestion/dyspepsia can be associated with actual impairment of digestive function. It can also just be a bellyache.
My mother was fond of calling me a dyspeptic when I complained excessively. Unfortunately, she never gave me HobNobs to shut me up.
Hey, I hadn’t know that digestive biccies were yet another great Scottish invention. 
Aren’t we wonderful? Of course, if I had given it any thought, the name just might have provided a teensy weensy clue. OK, I am dim.
The other reason digestive biscutis are a Good Thing is that they are responsible for leading me to the Straight Dope. Yep, nothing intellectual, just biscuits.
I have a friend, oringally from New York city, who lives in London. I think it was all because of some recipe she had that involved Graham crackers and she said what a pity we don’t have them here, and well, we had a highly erudite ) conversation all about Graham crackers and what was the nearest British equivalent, and about digestive bicscuits, and what the U.S euqivalent would be…oh we do have such riveting conversations, you know.
Anyway - apart from getting to read some hilarious stuff about Sylvester Graham, one reference that Google turned up for me led me to this esteemed place. Therefore digestive biscuits really ought to have the status of a sacrament.
Sorry for the hijack, Anyway,I have often suspected that the name is really only a fiendishly cunning marketing ploy. After all, “digestive” does have that "it is not greedily eating sweet things - it is eating something that sounds “healthy” ring about it, no? Perhaps the original recipe was different, but I have a packet of said comestibles in the kitchen, and I just checked the ingredients list and I can’t see that there is anythign very special. Perhaps 0.5 grammes of fibre is good for a sweet biscuit - I don’t know - but they are pretty high in fat. Oh well, I’ll buy more soon to do some further research. And surely sodium bicarb is by no means uncommon as a raising agent?
Oh, and we never did get to the bottom of the Graham Cracker/digestive biscuit thing. 