Do we need to enter and exit the UK at the layover in London, which lasts for an hour ?

There is a risk of not being able to catch the flight. It depends on whether you can bear this risk. If you can, since British Airways dares to sell, you can also buy.

I may not have enough time.

Even if the plane is on time, you won't have time. The plane pulls up to the gate, you get off, walk to the immigration checkpoint (unlucky, I've waited in line for 90 minutes before), get your luggage, check in and check your luggage, go through security, and walk to the boarding gate. Think about it...