why tell them lol just be carefull
Speaking as someone who manages the investigations into serious / fatal accidents I would say that is not a good idea !
Have a big one (your fault or not) and it will end in tears .... I have recently had a car examined with a view to seeing if it had been modified (it hadn't) but we were looking at the ECU and / or for a tuning box.
I am aware of three or four ECU's dealt with by us where they were found to be remapped and the drivers ended up well and truly in the poo ....
Is it really worth the potential grief for only another £50 or £100 a year ??
FWIW Greenlight for me on my Ed30 were no dearer remapped than Admiral were as standard .... at £495 (London; full NCB, no collisions, points, etc). The funny thing was they won't cover me at all for a remapped BMW 320d !!