American style vesting does not really exist in the UK in that format. And as such it's close to impossible to find legal agreements (founder agreements, SPAs) that cover such vesting. In order for this type of vesting to be enforceable in the UK, it's usually linked to good leaver / bad leaver provisions. This means you also have to have a service agreement or employment contract in place against which you can judge what is a good / bad leaver. Feel free to email me at ania at highskillpro dot com. Happy to help if I can.