There is just something amiss here to me. I am pretty certain that WC committed the crime as I genuinely believe the police would not have 'thrown him to the lions' if they weren't sure he was involved. I could cite many cases where the police have closed ranks to protect their own (Ian Tomlinson for one). What doesn't sit right is that if this is WC's first kill, surely given all of his training, it wouldn't have been conducted in such a 'sloppy' way (abducting someone on a busy road which is covered in CCTV with many properties looking out on to the road).
However, if this is not his first murder, why has nothing been flagged sooner? We've seen reports now that he has had a lot of time off recently due to "erratic behaviour", so if there is an iota of doubt regarding the mental state of a person who is armed for their job of protecting high ranking officials, why was he not immediately stripped of this role and either suspended or relocated to a desk job?
Was his sloppy kill perhaps an attempt at double-bluffing, where if he were to be flagged as a POI he could point to the fact that if he were to commit an offence of this nature, it would be more 'professional'? I think that this scenario (not necessarily SE specifically but the intent to kill a woman at this time in this place in this way and a carefully thought-out aftermath of disposal, alibi etc) means either a very drawn-out and meticulous plan to kill someone, or a moment of rage, either brought on by some kind of sexual reason (thrill or misogyny) or potentially abuse of steroids (MOO based on his bulging muscles). Could the erratic behaviour previously mentioned also be a result of steroid abuse? I would assume not as presumably officers in WC's position would be subject to regular and detailed drug tests surely but correct me if I'm wrong in that assumption.
Another thing which confuses me in terms of the psychological escalation is the 'path' from IE to murder. JMO but there is a large leap from flashing to murder. Is a more logical path flashing-sexual assult-rape-murder? If so, I think this lends a lot of credence to the article previously posted regarding the rape on Clapham Common last July.
Apologies for the long musings and further apologies if I'm just paraphrasing or summarising what many others have already said. There are still many unanswered questions about the case, including how SE ended up in the car (was she tricked or forced or tricked then forced?) and the overall question as to why someone would do something like this at all.