iOS doesn’t do what most people think on 5g.
iPhones will show the “5G” icon when the connected cell site has 5G or other local cell sites have it. It simply means 5G is available.
All of the Uk networks have rolled out 5G in an NSA / Non Stand Alone mode – which means devices remain connected to 4g for the purposes of negotiating how to transfer data.
For data transfer, network configuration will determine whether the data bearer is 4g/5G – so the phone may well actually be using 4g but still show 5g. 4g can now achieve speeds of 600Mbps and above on 3 and EE so a fast download and 5G icon may still be a 4g connection underneath.
At some point this year, uk networks may rollout SA / stand-alone mode for 5G – which will enable devices to connect directly to 4G first. Vodafone reportedly have a couple of cell sites operating that way for test purposes and I’m sure 3 / EE will be doing the same.