meic wrote:is that either British workers don't want them
It isnt that they dont want to do the job, it is that they think it is worth more in return for their labour than they are being offered.
That depends on the job. In the case of some jobs--picking soft fruit is a good example; it's hard, back-breaking labour, paid by quantity--British people mostly don't want to do it. Not enough to get the crops in, anyway. Some students will do it, and a few other people who are able to do that sort of work, and need the money, but not int he quantoty of people needed to get a harvest in.
We might be able to manage to get British workers to do that if seasonal labourers were paid an annual living wage for seasonal work. If, for example, they were paid £20 000 per year, and in exchange, worked very hard for some months during the year (planting, then May - September?), but farmers likely couldn't afford that, and food would become rather more expensive. It could well become cheaper to buy Strawberries flown from Morrocco, than those raised in Essex.
“In some ways, it is easier to be a dissident, for then one is without responsibility.”
― Nelson Mandela, Long Walk to Freedom