I know the Speedo/Odo accuracy of my Honda del Sol with stock size tires was poor- indicated was 10% higher than true. I changed to a larger diameter tire and now I'm much closer to true. (GPS self-tested)
Of course, this change has also affected the torque loads on the drive train and brakes as well as affected handling because the CoG is higher. I've done various upgrades to alleviate these, however.
In the states it's pretty common for SUVs and 4x4s to have much larger than normal tires. It's really up to the individual to understand the changes to their vehicle.
Are such tire size changes allowed in the UK? I know in Germany they are usually not.