If one were steelmanning the case that this was genuinely an accident arising from some sexual oddity, I think there are a few things one would point to in support.
First, he had apparently looked at BDSM websites.
Second, there is the episode in his previous flat where he had to be rescued by his landlady after he tied himself up.
Third, there is the collection of women's clothing, although I don't know whether e.g. credit card payments for these by him were ever surfaced to prove the garments were not just planted.
Fourth, he does not appear to have had any sort of relationship history with anybody.
Fifth, despite this being an MI6 property that you'd expect to be covered by CCTV, there does not seem to be any evidence of unidentified visitors to his flat.
The challenge to this line of thinking is twofold. One angle is that it's incomplete - it does not at all explain details such as the absence of an evidence trail leading to the holdall, or the heating being on during August. The other is that it could all be entirely irrelevant and used against him either for control or, later, to muddy the waters around how he died. Someone finds out about it and it conveniently suggests the plausible false trail to lay that points to a long-established pattern of behaviour. By analogy, if he had been a classic car enthusiast, he might have been found crushed underneath the vintage MG he was working on when its axle stands collapsed.
There was apparently no sign of poisons in GW's blood, but then, the samples were not taken for a week. Some drugs and toxins have short half-lives and can evaporate or degrade to undetectable levels in that time, or may undergo metabolic processes in the body post-mortem, so that they break down or transform into other substances by the time samples are collected for toxicology testing. Anyone who killed with poison would want the body to remain unfound as long as possible to give these processes time to happen. This would hide what type of poison was used, usefully so because that in turn might reveal when and how he was poisoned.
No killer would have any way to ensure he was not found short of taking him away and hiding him, which was not done. In this context, the complete failure of an intelligence agency to react to or even notice the disappearance of an employee is very troubling. I have a great deal of difficulty believing that was just an oversight, especially given the central heating detail. It's almost as though his employers wanted to make sure nothing showed up in any toxicology report and assisted this end by doing nothing to find him. Indeed, he wasn't found because of anything they did; he was found because his family became worried and alerted the police. If they had not done so, he would presumably have remained undiscovered for longer still.