Ah now come on, you can do better than that! A Polish 20 year old can afford to take the job, and so can a British 20 year old.
But why would a British 20 year old want to do it? There's no incentive. A job like that, for a British 20 year old, would be just condemning them to a job with no prospects and no future just eking by on minimum wage at susistence level, not able to have a decent life or afford a trip to the cinema or a takeaway on an Friday night. Just a permanent dismal life - no more money or freedom than being on out of work benefits, the only difference being that you're working 40/50 hours a week to be in exactly the same situation as you would be sat doing nothing. And they'd often have to leave their home/family/friends community for the privilege into insecure substandard housing in remote areas for an insecure low paid job. Why would they do it? Why should they feel morally beholden to pay into a country that doesn't give two shits about them?
On the other hand, a 20 year old Pole can view it as a short, sharp, shock of temporarily working very hard and living a parsimonious lifestyle. Because of the difference in value between our currency and theirs and the cost of living between our country and theirs they can save up enough in a short period of time to go back to their home country with a stash of cash which will set them up for life, pay a deposit or pay off debts. By contrast for a young British worker the same sum of money wouldn't even buy a second hand car let alone a deposit on a house.
Despite the popular left wing trope of hardworking industrious Eastern European migrants selflessly saving the country from bone idle natives; the truth is that people do something because there is something in it for them that makes it worthwhile. For a migrant there is, for young British people there isn't. It has nothing to do with migrants being inherently better, more noble and more industrious than British born workers.
Honestly, I think this con that the left wing has fallen for over the last 20 years is just astounding. Pre-Blair the Labour Party was concerned with the working poor, their rights and welfare, good working conditions, decent wages, decent affordable housing. But they've jettisoned all that in favour of prioritising migrants and migration over anything else.
The fact that people who honestly believe they are left wing will quite cheerfully lecture the poor on how they should be doing shitty, poorly paid jobs in horrible conditions with no future that won't even pay for a reasonable standard of housing is just so depressing. Leaving aside the fact that these supposed 'anti-racists' will cheerfully make statements about British people being lazy with no work ethic compared to other nationalities which would leave them spitting bile all over Twitter if made about, say, Jamaicans or Pakistanis or Nigerians.
No wonder so many people feel abandoned by the left.