HI Frank - It is a common thing for us to be denied access don't worry. Just for your reassurance it won't be too late inspecting your house after completion. In fact we do this most of the time.
But there are a couple of tactics you can approach in the meantime. Firstly, advise the site manager that you intend on using us regardless; as it is initially to try and put you off using us altogether. It is much easier for them to address 1 comprehensive list by us rather than addressing ambiguous lists they often get, and to do it with an empty house.
This is why it is not any company policy from any builder, because frankly we have been in prior to completion to every builder out there.
Your second option is more formal and that is to get the solicitor to incorporate it as part of the sale. In other words, you wont complete unless it is agreed for us to go in a week before.
I hope this helps and please feel free to follow up with any other questions