If Germany had offered a peace deal which included return of the POWs, the family and friends of the captured British soldiers might well have put pressure on the government to accept.
I very much doubt British politics of the era to work in that manner.
Any of the main political figures wouldn’t take up the campaign, the media wouldn’t run with the story and society as a whole was unlikely to look kindly on families holding the government to ransom.
We are at most talking of 200,000 families (of course allowing for soldiers with no families, brothers in service etc or even not all families joining the campaign) which, while it sounds a lot, will not represent a government-felling amount.