From someone who spent 10 years in the UK Payroll industry, good luck! I’ve spent many hours thinking about doing this myself.

One question that’s not clear from your website, what part of the market are you targeting? Payroll for a 10-person company is vastly different from a 10,000-person company (As I’m sure you are aware).

Regarding the API, personally I wouldn’t say the payroll processing API is a feature I’ve seen requested, but I like concept. The most common API requests are for employee data. No matter how good your integrations are, somebody is going to want names and addresses they can extract on demand.