Go on Fleabay, look for "Buy It Now" values for the items in question, that's your figure. Don't overthink it. If the worst happens you can haggle anyway, it's not as if they will haggle over the difference between £5k and £6k. If you told them it was £200 worth and then a claim said £5k, they would rightly complain. But £5k and £6k are the same numbers in their world. It's the same risk.
I did have my hifi stolen in 1995 but maybe the desirable items have moved on. Not many people had a home computer back then, I didn't. Then again CDs were worth stealing then, the various Theft Converter joints would give you £1 each so people would just hoover them up.
There used to be a school of thought that you should leave an item of reasonable value or some cash on view (on view inside, not visible from the window!) so that any thieves might well grab that and say "close enough" before leaving without further disruption. An old lappy and an old mobile phone might fit that bill.