Why has my acanthas not flowered this year after 4 years of successful flowering

Was the winter very severe where you are in the UK? I ask because we have the same problem following a severe winter. I'd guess the plant is damaged by frost and needs to make some more growth this year to produce the flowering buds for the next.
The other reason for many perennials not flowering is that they have become overcrowded, in which case divide the roots in the autumn or next spring and replant. But acanthus can get quite crowded without being affected in this way.
The site for the National Acanthus collection says:
"EA Bowles once said “Acanthus will sometimes sulk for years after planting (or replanting), but once they forgive you they ramp, while every atom of broken rootlet left in a two foot hole will sprout up into stronger plants than those removed”.

Plant Acanthus in a position where you want them to remain as once established, they are hard to eradicate! They make excellent specimen plants in tubs and pots.

As a general rule, acanthus grow in well drained soils - they don’t like to be sitting in water, especially during winter. They thrive equally well in both sun and dense shade, although they tend to flower more prolifically in sun and are slightly more compact whilst they produce more and larger leaves in shade."
You might be interested to see there are many different types of acanthus (I didn't know this!)
http://www.hillviewhardyplants.com/acanthus.lasso