Allow your staff to invoice you for their time. They can make it more tax efficient for themselves and you save on the National Insurance contribution. You should make it clear that they are not an employee and that you do not have exclusivity over their time and you should encourage them to take additional work on from other third parties.
Do not view that negatively. It is often said that one should ask a busy person to help with a task rather than an idle person. Someone who has endless time on their hands is often not the right man or woman for a particular job. Different work experiences add diversity and your business may benefit from it. It will certainly help with employee relations.
I have learned from one position certain skills and procedures and carried them over to a second job I had at the time, much to my second employer’s gratitude. We are all learning, it’s an ongoing process, for us personally, professionally and for business entities.