Mulberries are not really a fruit that can be bought easily, you have to know where there is a tree and go picking.
Mulberry trees across the UK are largely thanks to King James I who wanted to create a prosperous silk industry to surpass the French. The silk worms sole food source is the mulberry, however, it seems that the King was misled, some say by the French, to import 10,000 black mulberries when it is the white that the silk worms prefer.
This turned out to be a blessing in disguise, despite the failed silk industry, as it is the black mulberry which is the tastiest with flavours somewhere between a blackberry, raspberry and a tart plum.
The trees where distributed to wealthy land owners across the country and while today many are still on private land, they are also found on the grounds of old estates which have been turned into parks or have public foot paths running through.