Standard car insurances will start with a Glass's Guide figure and adjust for mileage as per the guide. I've not looked in one for years but you might even find the Z3 is out of it due to age, I can't remember how old they go. The fact you put £5k in as a value won't override it.
Agreed values do tend to be with more specialised policies like a classic car one, rather than the avarage "tesco.com" policies. My brothers car is on an agreed value classic car policy and for the last few years the value has risen significantly. Each year he increases the value and pays a bit more, but the total loss value is fixed and agreed.
You do need to be able to justify the higher value in the event of a claim though. You can't just say you think it is worth £20k, when an average one sells for £2k and a real nice one makes £4k, and still expect £20k! We could all make a nice living out of that.
I had a no fault accident some years back and my car, which I was trying to sell at the time for £3k, was written off. The other chaps insurers offered me £4.2k for it

. I still asked for £40 for the diesel and got that as well. Rest of the claim went a bit sour though

.