As always with these things there is a threshold to meet which is generally what is reasonable or reasonably obvious to a layman. What defines reasonable is always a bit of a grey area and it will vary from case to case but say a set of wheels which are OEM but were not fitted to that particular car at the time of original sale. How are you, a reasonable layperson, supposed to know that? You aren't likely to, so you would generally pass the reasonable test. If you have a 6" wide exhaust pipe on your Corsa 1.2, it should be clear to anyone that it is not standard and thus you would fail the reasonableness test. Very visible, brightly coloured air intakes are not likely to be reasonable. Remaps where you have not yourself remapped the car and have no knowledge of the presence of a remap would also probably be OK - you're not an expert, you can't interrogate the car's ECU so as a layperson there is no way you are going to be reasonably expected to discover and thus declare it.
I'm not a lawyer so take the above with a pinch of salt. In the case of the OP, simply having asked for a quote for the modification is clearly enough to prove that the OP was aware of it, so either put it back to standard, sell the car or find a mod-friendly insurer.