I've just tried to make this recipe from the BBC Good Food site: https://www.bbcgoodfood.com/recipes/simple-soda-bread.  The only variation was I used semi-skimmed milk (2% fat) instead of the whole milk specified.  The result on mixing the wet and dry ingredients was a sloppy, gloopy mess, completely unsuited to be "shaped into a ball".  Was it because there wasn't enough fat in the milk, or is there likely to be another cause?  
The fat in the milk is a red herring here. At best, you would have to be a professional baker who knows the same recipe by feel to be able to notice the tiny difference made by 2% vs. 3.8% milk in that recipe. I would start in very different directions. First, was it really a disaster? This is a 80% hydration dough, and that with whole flour - I would fully expect it to be sloppy and gloopy. There is nothing wrong with that. If you haven't baked bread of different hydrations before, or no soda bread (which has lower gluten and so doesn't come together as much), you may be imagining something firm and plastic like pasta dough. This is not what the recipe is about. When they say "shape it into a ball", they don't mean a sphere, they mean a boule. It will be certainly wider than it's tall. Just continue working with it. If it is too extreme - e.g. it becomes a flat disc, or even flows freely - I can think of two possible problems.  maybe you didn't measure correctly. The recipe is given by weight, but if you tried to measure by volume, you might have been off. Or you were not perfectly concentrated and by mistake put in 500 ml milk and 400 flour, or a similar mistake. maybe you used a different flour than what they have used. While white flour is pretty standardized, whole flours can differ a lot in their absorption ability. Update: As per J..'s comment, the amount of lemon juice can be significant for that amount of flour. It might be worth using a smaller lemon, or not that much juice. (Or maybe go with real buttermilk instead of creating a substitute on the fly).  In any case, you can try saving the failed recipe by adding a little bit more flour this time (but don't make it as firm as mid-hydration yeast bread dough). 