I can confirm that yes this is a bug, our postcode checker is falsely rejecting it! I'll get it fixed soon.
Joe - Voice Admin
As far as I can tell, the problem is actually just a typo.
You were trying to use BN7 2Q5 (ending in five)
And the correect postcode for the town hall is BN7 2QS (ending in the letter s)
I tried the latter and it works fine!