If your company is registered in the UK then market-standard practice would be for your investment documents to be governed by UK law. This is commonly accepted by investors from all over the world.
The company they are investing in will be subject to UK laws and regulations, the majority of employees are likely to be UK based, the funds invested are likely to be held (and in most cases spent) in the UK, and generally UK law is viewed as a good standard for international transactions.
There may be limited circumstances in which the law of another country would be suitable, but you should discuss this with a legal advisor with suitable qualifications in that country before committing. As a default, we would say that all investments into UK companies, and all documents relating to those investments, should be governed by UK law.