Going to get a roasting for this and happy to hear your opinions..... The moral argument is difficult as the more people who come into the country the greater the pressure on infrastructure which could have negative impacts on the current population. For example, increased demand for healthcare and medicines means less money per person which reduces the ability to provide quality healthcare, so reduced quality for all.
If there are more people who can do the low paid jobs (not all migrants I know are low skilled but a large number are) means minimum wage remains stagnant as we don't need to provide incentive for people to do the jobs no one really want to do.
Increased demand on schooling means less time/money per pupil so quality of a schooling reduces which could impact on someone for a lifetime.
It's great to be humane and deeply care for the welfare of others, it's what makes us human, but really you can't say on one breath we should let anyone join us and in the next complain your child hasn't got a space in the local school due to lack of spaces or complain that you can't get a doctors appointment for 3 weeks, or your hospital doesn't have enough beds.
We have nearly 10 million more people here than in the year 2000 and it has knock on effects unfortunately, majority of which is caused by migration. We have increased numbers by 17% but we do not have 17% more schools or hospitals or gps. We can't support everyone, so the numbers coming in should in my opinion be kept at a sensible level. To determine this I think really you would have to establish the basic quality of life and services you would want per person and only increase population at a rate to not impact this.