If the company has created a prototype that's received $150,000 in crowdfunding support, that's a tremendous accomplishment. The existing founders have conceived of a product, prototyped it, and validated it in the most significant way possible - with customers who are willing to not just pay for it, but pay for it far in advance.
Let me put this in perspective: on Kickstarter, raising $100,000 puts a company in the top 0.2% of successful campaigns [1]. If you include all campaigns, they're rarer than 1-in-a-thousand. Due to selection bias, we hear about these >$100k campaigns preferentially, but the company is breathing rarified air. That indicates there's real value that's already been created by the founders.
That said, 2% and a smidge over minimum wage isn't right either. The compensation hit you're taking is so significant that the equity position should be larger. I would be looking for something in the 5-10% range. The question I would ask them is this: the accelerator put in $50k. You're forgoing much more money than that. The accelerator got 6%. Do they expect you to provide more or less value over the next four years than the accelerator did in 3 months?
Also, while verbal promises are legally binding (in the US and I believe in the UK as well), they're hard to prove. The suggestions to get the "salary after funding" agreement in writing is a good one.