Its a big reason why I can't do it. I have bills that must be paid. Student loans and they are larger than most everyone I come across.
It's kind of sad that startups are a game for people who have a long personal runway (combination of savings and cheap lifestyle, usually), but that's the reality.
No investor is going to be excited to see their money put to work by paying down student loan bills, though at a certain level you can often swing a 50-70k salary. As I understand it, get big revenue or a big Series B round, and you can generally lobby successfully for a non-insulting salary.
But your salary needs are a reflection of your commitment. i.e. if 3 founders raise $500k, $50k salaries give them a ~15k/month burn rate fully loaded, approximately. $100k salaries give them a $30k/month burn rate fully loaded. The difference is the ability to hire two solid ppl.
Which do you choose? Is your lifestyle more important than 2 hires? Alternatively, if you don't need to hire any more, how much runway do you shave by making $100k across 3 founders?
Investors also want interests to be aligned. Low pay early on means that you're motivated every day for growth (because revenue or a larger investment round means a raise). They'd certainly rather see their money invested in growth than in founder lifestyles.
Of course, every situation/investor/founder is different... But yeah, expect to take a big pay cut.
If you have less than a million in the bank, I think the question needs to be framed as "How much do I need to survive" and NOT "What did I make at my last job". If the founders aren't trying to dig deep here, then I'd wonder about their motivations (unless revenue was taking off). Lack of runway kills startups.
There are no IPOs these days so at a typical .5-1% the best outcome for a hired engineer is for the company to get acquired in < 2 years, make about $300K and get a job at the acquiring company that pays 2x they would make if they just went to work there in the first place. Kids who go to Stanford and MIT seem to have this figured out and are expecting to be paid market rates. Your market rate may vary.
The worst places I've worked are the ones where the founders conned all the staff into working for $50-$70K by using extremely optimistic outcomes for demonstrating how much their options might be worth in the future. Which is like 70% of all startups in Silicon Valley.
EDIT: as a corollary, I'm thrilled I didn't pay off my student loans earlier. They average about 4.5% - that's the cheapest loan you can get anywhere. Treat it like a business loan and use whatever you save to live off of when you're working on your startup :).
The other aspect is that UK student loans are probably much smaller for the majority of people.