not sure bout the differnt parts of the uk, but here in nz you can get fruit within about 4 years.
just to note, there are at least 3 kinds of mulberries (types not cultivars - mite me untols numbers of those)
there is the white mulberry that grows Uber fast - ours ahs been in 4?5? years and is well above head hight ATM despite being "pruned" yearly, with a chain saw.
white mulberrys, are IMHO a waste of time except maybe as stock food - they are heavy bearing and INTENSLY sweet - might be good for making vodka i suppose. folliage makes good brouse for sheep (and maybe others). Have heard they can grow to 40 M - dont know if that is height or diameter.
Reds - two kinds - dont know the distinction, just know there is one.
We have recently planted 30 odd 15inch/45cm plants - DO use snail bait around them untill large - it is not just silkworms that love the leaves.
With a good bit of feeding 5-7 years should see them fruiting regularly give them 10,15-20 and they will get 10-15 Meters (30ish feet?) high and about the same accross, and provide more mulberries for jam, wine, dying and bird food than you can possibly use.
Have also read that in the USA pig farmers used to grow two trees - mulberries and acorns and that, with the grass and a minimum of food would fatten 12 pigs +.
They got litraly tons/tonnes? of berries from a single tree alone (admitedly an Old - 35-45 year tree fertilized with pig maure

), and many planted forests of them . . .

Jeremy Daniel Meadows. (Jed).
Those who walk in truth and love grow in honour and strength