Assuming this is private sector, holidays should be approx 24 days plus bank holidays. Not much difference between companies and not negotiable (except in startups where unlimited holiday is becoming more common - not as cushy as you might think).
You lose your current options and may or may not get options in the new role but they’re unlikely to compensate you for the lost options unless you’re in a very senior role and are headhunted into it with golden hello’s.
Looking at your salary, it will be hard to overcome that you’re so underpaid currently and they might try to offer you a sizeable pay rise to take you up from what you’re on now but will still be below market. Employers often look at hiring people for what they can get away with, not necessarily what the job is worth. It shouldn’t be like that but it often is, even in huge corporations.
The agency will know at least what the potential salary range is, be armed with this and go in and sell yourself. Regardless of what the role is your interview is a sales pitch for yourself.
Play up the value you’ll bring to the role, how you are the best person to grow the business and you deserve to be compensated accordingly. If they ask what you want to be paid ask them in return what the job is worth to the company then state your case.
Especially with the business development angle ask them what the target is, what the bonus will be when you hit it and then tell them that you’ll smash it and want kickers when you exceed it, which you will.
Know your value and don’t shy away from demanding it. Be prepared to walk away if they don’t offer what you want.
It’s a negotiation and you need to have a red line or they’ll take advantage, it’s their job to do that.
Don’t accept less than you want in the hope that they’ll increase it in time, as you’ve seen in your current role it’s really hard to overcome starting on a below market salary