The UK police interviewers may have been confused, but there's no evidence that the PJ were. I don't know why Gerry said he unlocked the door with his KEY and neither do you, but he said it.
Perhaps the single sensible credible reason why Gerry made the (not at all) ridiculous claim - of going in via the front door, was that he went in by the front door? That's more credible to me than the proposal that he forgot what he'd done the evening before and every evening since Sunday.
The bit of your post I've bolded is your opinion and cannot be verified.
We shall have to agree to disagree then G - as I don't believe Gerry did say it. IMO he may have said 'the front door' but was talking about the patio door. I believe he and the PJ officer were genuinely at cross-purposes over which door was which.
For what possible reason would he decide to say he went all the way round to the front door - after already confirming that the patio door, a couple of seconds walk away, was open? He would need to have a good reason for making that claim - as it doesn't make any sense for him to do that - (especially if he needed the loo).
Sorry but as you give no reason why Gerry would suddenly decide do something so inexplicable - then just giving ''he went in by the front door'' as a credible reason doesn't work for me. What was his motive?
It seems clear to me that the PJ officer was not familiar with the layout of 5A or the immediate area - because if he was then surely he would have asked Gerry the obvious question i.e. ''WHY did you walk past the open patio door and go all that long way round to the front door?'' during that interview.
If he had - then I think any misunderstanding about the doors would have been cleared up there and then.
AIMHO