Even if the business you invest in is successful, it may take several years to get your money back. You are unlikely to be able to sell your investment early.
The most likely way to get your money back is if the business is bought by another business or lists its shares on an exchange such as the London Stock Exchange. These events are not common.
If you are investing in a start-up business, you should not expect to get your money back through dividends. Start-up businesses rarely pay these.