Firstly, work out how much your 0.2% equity is likely to convert into. You'll probably pay income tax and employers NI on them (and in one year!) and so you'll likely end up paying 55% tax on them. How much do the founders think the company is worth right now? If it's close to £500M, that's a big part of your comp. If it's close to £50M, it's not.

Next, work out where you want to be in terms of comp in a few years, rather than thinking of how to optimise the cash right now. For example, I'd stop worrying about your tax-free allowance gradually disappearing, and instead try to work out how to get it to all be gone. In 5 years, the person who makes £120k is £40k better off than the person who makes £100k, after tax. That... sounds worth it. And it's usually easier for the person who's getting paid £120k to get paid £130k than it is for the person who's getting paid £100k. This is to say, having high tax brackets is a benefit, not a curse.

And then, it's probably worth noting - this isn't directly their money, especially if they're looking to sell. It's probably worth having the conversation of like, 'what would I need to do in order to justify £100k/year?'. Or, alternatively, negotiating on the vesting of your stock, since that's effectively free. If they think the company is going to be sold in the next few years, that's a relatively small giveaway for you. If the company's grown a lot in four years, it's unlikely a significant increase in stock is on the table.

Don't overestimate how long it'd take a good new person to catch up. I've rarely seen a role where a new person can't be effective within 6 months.