I've heard that student loan debt is not considered a derogatory
factor when being evaluated by a mortgage lender. Is that true?
I'll focus on this part of the question since there are plenty of answers for the rest. I'm going to assume that by "student loan" you mean the official government-sanctioned one administered by The Student Loan Company and not just a personal loan from a bank aimed at students.
Student loans do not appear on your credit history. There are numerous sources claiming this, but I can also confirm it personally as I have a student loan and am in the habit of obtaining my credit report from all three main agencies (Equifax, Experian, Callcredit) on a regular basis: both the statutory reports and the online ones. In 16 years it's never appeared on any report.
However, that doesn't necessarily mean that it is irrelevant for a mortgage application. The majority of applications will ask you to provide a list of your assets/liabilities and your income/outgoings. If you fail to mention your student loan then it is entirely possible your lender will never find out, but it could be considered fraudulent (which if discovered, will negatively impact your application as well as future applications, as lenders also report to fraud agencies as well as credit agencies). One possible route to discovery would be if the lender asks to see your tax returns (not uncommon these days).
Primarily this information will be used to determine affordability. Not all lenders treat all outgoings the same, and the regulations are constantly changing. So it's best to discuss this with an experienced mortgage broker. From what I've read, affordability is the only issue and lenders won't consider it a "black mark" that you have a student loan. Note that at £10 per month, this is so small that you might as well consider it irrelevant from an affordability perspective.