The only thing that selecting pre-2000 or post 2000 in Heat Punk changes is the air change rate assumptions and MCS guidance is very clear that air change rate values do not necessarily reflect the age of the property and that the installers engineering judgment can over ride these default CIBSE values.
See what they say if you are very specific that you want them to use lower air change rate assumptions. E.g in Heat punk select pre-2000 even and then manually change the assumed air change rate in each room to ~0.6 ACH…
If they are not receptive, perhaps it’s worth trying a different installer?