You don't take the UK government at it's word. You take the leaked cables at their word that cover renditions in Sweden and other countries, but not the UK.
Which leaked cables establish that there were no prisoners on board rendition flights passing through the UK? And moreover, establish that this was a matter of policy?
Zooming out a bit, there is very little analogy between the Agiza/Alzery case and anything that could conceivably have happened to Assange. Ahmed Agiza and Muhammad Alzery were asylum seekers whose asylum applications failed and who were then deported to their home country. The correct legal procedures around deportation were not followed (and of course the Egyptians broke their promise not to torture the two men), but it is a huge stretch to try to make an analogy between this case and some supposed plot to kidnap Assange once he was in Sweden. The latter would have been a vastly more flagrant breach of the law than a rushed deportation, and would have made a mockery of Sweden's original extradition request.
The simple fact is that it is extremely unlikely that the US would have kidnapped Assange from any location.
