40 years after you take out the SFE student loan (or by state pension age, whichever comes first) it's written of in its entirety. Combine that with the fact you only ever make repayments while earning over the threshold, the repayments are proportional to your income, and if you are a PAYE employee (i.e. most salaried employees and many non-salaried employees on contracts with hourly pay) you don't even need to do anything, it just comes out with your income tax, NI contributions etc. It's almost impossible to default on it (basically the only way would be if you misrepresent your earnings as a self-employed worker, or leave the country and don't tell them) and doesn't affect your credit score etc. Literally student loan "debt" is the absolute last thing that should be a decision factor.
You can read more about it here:
https://www.gov.uk/government/news/8-things-you-should-know-about-your-student-loan--2I don't really understand your reasoning of picking psychology because you preferred English but in any event, provided you have no issues with continuing to do maths throughout the course, not an issue there.
There's really no downside to doing a foundation year...being familiar with the campus and local area can actually be a benefit. Worth checking
where the foundation year is actually based/taught - sometimes they base them at local FE colleges which can change the experience a lot. Also worth checking what the progression criteria are and how many students progress successfully (both in general and to their desired degree programme - particularly notable if you aren't guaranteed a place on your desired programme if you meet progression requirements!). These are things worth checking, although I'd imagine Manchester and Birmingham at least will have a decent set up?
Since you note the Durham foundation year, I would guess you're a widening participation/contextual applicant? If so then something else to consider is that a
lot of foundation years have significant bursaries for the foundation year itself - usually a combination of partial tuition fee waivers (so you don't even have the full tuition fee being paid by the loan, although as noted above this is a bit of a moot factor and not as good as the money in your pocket!) and 1 year enhanced bursaries for the foundation year (and then you get whatever you would normally be entitled to for the rest of the course.