You don't require planning permission once you don't deviate from the character of a property as you have alluded to.Did you seek planning permission or get a certificate of exemption? I thought planning was required for a significant alteration to the character of a property.
Might become an issue if/ when you come to sell and the prospective purchaser's solicitor seeks confirmation of planning compliance.
Once the external insulation has been applied, there are almost endless finishes that can be applied to the external render. One such option is "brick slips" where very slim real brick segments are fixed to external render and the appearance can be almost exactly as previous. Here is an example, which is just the first result I found on google. https://www.abac.ie/photo-gallery/slip-brick-system-external-wall-insulation/
Similar options are available for pebble dash, various stone finishes etc. So if you're in an existing red brick property, you can end up with an externally insulated property which very closely matches the original and is thus in keeping with the character of the propety.
If you're in a listed building, you will need planning permission in any case.