"Research shows that students tend to stay where they study or move nearer to family upon graduating which means fewer will study and stay in major population centres like London. That affects public services. "
The usual complaint is that London is acting as a magnet and pulling in a huge proportion of the population. Why is something that has the effect of spreading the population out a bit a bad thing? You appear to be arguing that not merely is one current effect of education to condense people into major conurbations, but that it's a good thing and should continue. If fewer people crowd into London, house prices there will fall and the massive London-centric movement of jobs to the south east would stop. That's not a bad thing, surely?
"Many people have pointed out that the debt is the child's not the parent's but who wants to see their child struggling to have a decent standard of living well into their late 40s and 50s? "
But no one is remotely suggesting that. The thresholds and tapers mean the contributions are small until people are earning above the national average, and if they don't reach that point, it's written off after 30 years anyway. The struggle comes from the housing market, not the price of education, and if I suggested that what this country needs is a 50% drop in housing prices I doubt I'd get widespread public support.
" Those who are considering the impact of huge debt on those future choices will 'shop' for a cheaper degree in a more affordable location. "
Perhaps they will. Again, is that necessarily a bad thing? A university where accommodation is cheaper than SW1 will have an advantage over Imperial, and may be able to attract good students with that. A university based somewhere with lower staff and property costs - oh, I don't know, Bangor - could offer top quality degrees at a lower cost than UCL. Is that necessarily a bad thing?
Free tuition for all isn't going to happen, unless you want to return to take-up rates (and, let's not forget, the size and number of institutions) of the late 1980s. Given that, you need to consider whether the current system, which attempts to pretend that degrees all have equal value (which means that people whose parents and schools are smart enough to point them to the RG get a bargain, and people who end up at random ex college of FE X pay over the odds even at current prices) is the only possible option, if there could be a better way.
Let's accept that a real, underlying cost of a degree is about £25K. That's intuitively right: it means a year at a university costs about the same as a year in a school. Should that be funded by the state? How much do you want to put on income tax? If not, where should the money come from?
"It also creates a system whereby students who do not have to worry about future debt can apply to the more expensive and more highly regarded universities, re-inforcing the current system whereby decisions made which affect the bulk of the country's population are made by upper middle class men with Oxbridge degrees and healthy bank accounts."
The rate of people from state schools attending Oxford has fallen over the past thirty years, from (from memory) something like 60% to less than 50%. It would be profitable to figure out how that happened. It's not about money.