Depending on the network coverage in your location, you could experience a range of different data speeds. 

Data reception and speeds may not be as good indoors or in a car. The signal can also be affected by buildings, trees, and even weather conditions. You can check the current state of the network online, here: https://status.o2.co.uk/ 

Cell site congestion can be an issue at times, more so in densely populated areas, but any drop off in network performance is usually temporary.

To ensure the best possible experience, take steps to correctly configure your phone's mobile data settings. Follow the instruction in the guide for your operating system: https://www.plan.com/apn-settings