Early on in your career, don't have to stick with big 4, take risk, join the 10 yrs old UK based company, learn something new.
Because early on in your career, you do not have loans to pay (I am assuming this one), you do not have wife or kids that you have to give time to, you're not paying huge rents, you're not paying house/car EMI.
Use this period, take risk, learn something new, switch again in 2 to 3 yrs, repeat.
Having said that, even though it's a startup don't burn out yourself, if you see that happening, switch.