Ask HN: Where to get tax advice as US-UK migrant on US/UK stock option exercise?

I'm a US Citizen living in London, so taxable by both HMRC and the IRS. The past few years, I've been working as a mid-level software engineer for a British YC startup. It was incorporated in Delaware but mainly operates via its entity in Britain. Within 90 days of leaving, I exercised options that I earned by doing work entirely in Britain. I paid dollars from my US bank account and received stock in the US entity.

I'd like to be more certain of the tax implications of my choice to do so.

How would I best search for where to get advice on this?