I run a start-up so I'm biased. I also used to work in academia, and went straight from that to a start-up because I loathe the idea of working for a big engineering firm (I'm an electrical/electronic/photonic engineer). So keep that in mind when reading.
Some comments here point out that only large organisations have the infrastructure required to really pull off certain fields, e.g. ML. Agreed, but surely this is field/interest dependent. I think most fields can be supported well by a start-up environment.
Consider the productivity. Some of the best engineering and commercial achievements were done on a tight budget - see the history of ARM, Microsoft, etc. I can vouch for how demanding, educating, and satisfying it is to work in a closely knit engineering team towards a common goal, with everyone around you pulling their weight and having their talents 100% utilised. There is no room for slackers in a small organisation - you will not have any useless colleagues, no wasted time, no pointless tasks. Your skills will expand in every direction - and not just in technical fields (although you will probably need to dabble in a range of these).
Consider the share options. 6% is a lot. Have they raised any money? Find out. Imagine they've raised £0.5m for 20% (a small angel round). That values your 6% at £150,000 today. If they raise a Series A of £2m for another 20% in a few years' time, your shares will be simultaneously diluted by the newly-issued shares but also stepped up in value. Now your options are worth £0.48m. By the time you exercise your options, you'll probably be a millionaire. This isn't trivial, and it's a typical conservative trajectory. Of course, you need to consider the 6% offer as an investor would - do you think the business is going to succeed? If not, go to Google. If you think it will, why turn down that transient opportunity?
Consider the transferability of the skills you will develop. If you work for a large org you will focus on one field, probably. You may become renowned for that. If you want to work in a start-up later, you need to find the one that suits, and they better be able to afford your specialist skills. If you work for a start-up now, you will develop broad skills and a huge amount of experience solving multiple challenges. Your engineering confidence will be immense and this will be recognised by employers. A very capable generalist who can be trusted to learn whatever is needed to solve tomorrow's challenge is every engineering employer's dream.
Do you want to be an anonymous cog in a corporate machine, or do you want to be a genuinely valued member of a team that is producing something new and valuable, that is creating its own culture, its own processes, its own market?