Ah, but this is the Frozen North. Actually, there is a fair bit of differential in the UK. Blackberries are usually well in season in July in the south of England, but everything happens a bit later the further north you go.
I wonder if that procedure applies to mulberry. We have mulberry bushes in our extended garden and my husband had them pruned 2 weeks ago. Yesterday I noticed that buds have sprouted, so many of them and they surely would turn into fruits. Since mulberry is a berry that cannot wait for long to be eaten (not good if not fresh anymore) I had been thinking to make a jam out of it. And the only obstacle is that we have no idea on the recipe.
I'd try it with the fruit and some sugar on the basis of nothing ventured, nothing gained. If it doesn't work out, all you've done is waste a little sugar. We tried making some elderberry jam once and it was, in all honesty, pretty rubbish. There's never any harm in trying, though.