Fuck that. Why should Assange get special treatment?
It's also not legal. The Foreign Office do not have the power to tell the police who to arrest, or not to - and nor do they have the power to tell the courts to issue a particular judgement.
Not that "quiet words" can't be had - but why waste political capital on this. He can just sit there and rot, until he comes to his senses, or he's waited out the Swedish statue of limitations. Particularly as it sets a bad precedent.
On the flipside, not only can the FCO not tell the police what to do. But they also don't have to pay for police budgets.
It appears that Blair's governmet did illegally give letters of assurance to certain IRA people that they wouldn't be prosecuted. Which we know about because one bloke used his a couple of years ago, even though it had been given to him in error. However that was done in the cause of peace in Northern Ireland - which is important. Julian Assange is not.