M40 - stable banking job but looking for a role in a startup

I'm on the older crowd here and to make things a little more complex I'm in the UK rather than USA.

I work in one of those gigantic US Banks in London and I have a decent salary, good bonus, good private pension and very good private insurance for the family (pls remember - UK based, not US!) and the usual 28 days annual leave but the job is very stressful with long hours.

I am at that point of my career where I started to look around and found this role that seems a perfect fit but there is this disclaimer at the bottom of the job posting that says something like 'the initial remuneration during year 1 will be mostly made up of month/quarterly bonuses/add ons with Equity'

I'm a bit lost here so my question is - is that normal for start-ups to pay that way? If yes, what should be the premium vs. a stable Banking job? What should I ask when I talk to the agency / employer? (also please remember that UK salaries are significantly below the US counterparts)

Thanks!

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