Do not see what difference it makes whether the police entered WHF straight away or not. Everyone was dead.
For Jeremy I suppose the longer it lasted, the better. He could play the worried son & spend longer implicating his 'nutter' sister. The more dramatic it is, raid team etc, the more it looks like there is someone dangerous inside. Jeremy himself claimed he saw movement inside, or was that Bews ? Either way it was agreed it was a trick of the light.
If Jeremy was planning this, he would know that the police would get the raid team in when there is a gun/hostage situation. Meaning a long delay while Jeremy was innocently outside WHF with the police.