I thin that we have to also look at the "cost" of the value added by immigrants in, to take your example, the delivery sector.
No doubt many foreigners work in this sector, in which I will also add Uber as they are now in that sector - in over 100 call outs I have been picked up by over 100 foreigners, never once a clearly obvious "white english person".
I don't care who performs the service, for so long as it is performed satisfactorily.
Why is this? Is it that the native workforce is too lazy, or unqualified. I suspect the former and not the latter. But the other factor is that migrant transient workers come a lot cheaper. They are prepared to work for less money - perhaps due to not paying tax, or perhaps due to lower expectations and lower costs of living "back home" where much of their remuneration is heading.
So employers get a better deal and hence the (largely) unskilled migrants in this sector add value - at least they add value to the employer. But what about "society at large". I now a few people who are in this gig economy whilst furloughed or redundant or just people looking for a first job / students - and can barely get a look in because they are priced out of the market. An expectation of £ 10 an hour is laughed at - offers of between £6 and £7 are more likely. Doesn't even cover the vehicle and insurance costs unless you get 50 hours a week.
So yes, the immigrant workers are useful, and cheap. But that means the native workforce is then on state benefits, so add that into the equation and, assuming (and its a big assumption) that the home work force would take up these jobs if there were no migrant workers, Im not sure its good for UK plc, even if it is for the employers.
Clearly this does not apply to all, its just an example taken from rA thread. Cheap imported unskilled labour isnt useful to UK plc despite being a boon to individual employers; imported skilled and semi skilled labour is, and the ability to select what UK plc wants is a positive.