Your post offers sound possibilities to consider. I would add another. What if there is a crazy, or malicious, employee at Porton Down, who is doing this?
Originally it was thought that both of them had gotten sick from heroin overdoses, which meant that they were known drug users. But they were also “dumpster divers” and I suspect that they bumped into something that somebody - or some agency - was trying to throw away.
Britain has had its share of serial killers...