What laws should I be aware of (in the UK) if I build websites for multiple clients using the same template?

If I were to charge a client to build them a website, I'm concerned that a paying client would own the IP to the site, and therefore, I cannot sell another client a site using the same template.

There are many out there doing just this, but is it possible that a client could potentially sue you?

For clarity, I'm not envisioning on having my clients sign a contract, but perhaps I should do, which would include a statement that clarifies they don't own the IP to the template?