If it's an OEM option retro fitted - I do not declare. Would you declare factory fitted bucket seats worth £20,000 on ebay?
If it's a safety improvement - eg brakes, hoses etc - I do not declare
If it's a performance enhancement, eg larger turbo - I would. AS I would for LPG or NOS
That said the last very modified car I owned (Evo VI) I declared everything except the carbon rear spoiler blade (I forgot)
A-Plan used to say if it doesn't add 10% fro factory - it isn't a performance upgrade. They also classified the Evo remap as "UK fuel calibration" their words , not mine 🤣