I did exactly that many years back. I had an 850cc mini which cost me £20 and because a mate used to rally them I bought some bits of him and throught it would look cool if I fitted his old rollcage into the car. The car only had about 18bhp as stock, and with the heavy old cage it made the car even slower. Safety wise, it made it safer, but for me it was mainly the looks. Even though it made the car significantly slower, the insurance companies wouldnt touch me. In the end the cheapest quote I got was nearly double the insurance I was already paying (£500 ish third party only), so I had to remove the cage.
It is all a grey area really. Like others have said, once your consumable bits wear out, how many people still have OE brakes, tyres, exhaust, oil filter, etc. etc. You tend to have whatever the garage fits. Does the insurance company care if you fit falken tyres rather than bridgestones? In practise probably not, but they could do I guess if they get upset about it.
The only car ive had written off and an insurance inspector come out to see was a Peugeot 106 diesel which someone piled into the back of me and wrote my car off for me. The interior was totally worn out, and a set of seat covers were going to cost me about 30quid. So, for £25 I bought the entire interior out of a Peugeot 106 GTI and fitted that instead. Simply because it was cheaper, as well as the seats were more comfortable (sporty bucket ones). Insurance company didnt even comment on it when they came and did the appraisel of the car.