Springs as you are limiting the load carrying ability, regardless of your best intention to never overload. You can have vehicles reclassified for things like this but by the without doing so, it could get sticky in an investigation.
Check your chassis plate for the axle weights. Half the higher value and then calculate the tyre rating required. T32 is normally 1710/1720 so 103 rating should be enough at 875 kg (1750 kg across the axle).
Some info here including a chart for load ratings on page 1.
Thanks for that, looks like mine is OK at 103 rating:
ThanksAs long as they're correctly load rated, 235/55 will be fine and almost identical in circumference so no change to the speedo.