Our daughter will be traveling to England for a few weeks and would like to take her laptop with her. I'm guessing it's not a good idea to use a power converter for an extended period, but we have seen a power cord available online that will connect to the power box which also has a UK plug.

Are they safe to use that way?

Yes, most UK computer stores will be able to help.
You'd need a complete power supply unit as it will convert 240v to the required voltage for the laptop. These vay depending on brand, which you missed out.