I didn't say the permutations of assault were interchangeable, but that they are a spectrum of charges for similar offences - attempted murder and assault with a deadly weapon, for example, are quite similar, but the former charge requires proving intent to kill. I can't find the actual Framework for Plea Negotiation document right now, but to quote a QC "Prosecutors have always had the power to accept pleas to a lesser offence or offences charged if they consider that it is in the public interest to do so." Yes, if the fiscal (or English equivalent) believes they can evidence the necessary elements for the more serious charge, they will try to secure a conviction on it.

In fact, in a sense we come full circle here: in researching that, I came across the House of Lords Judicial Committee (McKinnon v. Government of the United States 2008 UKHL 59) - the case which triggered this family of threads in the first place:

"It is as well to recognise that the difference between the American system and our own is not perhaps so stark as the appellant’s argument suggests. In this country too there is a clearly recognised discount for a plea of guilty: a basic discount of one-third for saving the cost of the trial, more if a guilty plea introduces other mitigating factors, and more still (usually one-half to two-thirds but exceptionally three-quarters or even beyond that) in the particular circumstances provided for by sections 71 – 75 of the Serious Organised Crime and Police Act 2005 – see R v P; R v Blackburn [2007] EWCA Crim 2290. No less importantly, it is accepted practice in this country for the parties to hold off-the-record discussions whereby the prosecutor will accept pleas of guilty to lesser charges (or on a lesser factual basis) in return for a defendant’s timely guilty plea. Indeed the entire premise of the principle established in Goodyear [2005] 1 WLR 2532 is that the parties will have reached an agreed basis of plea in private before the judge is approached. What, it must be appreciated, Goodyear forbids are judicial, not prosecutorial, indications of sentence. Indeed, Goodyear goes further than would be permitted in the United States by allowing the judge in certain circumstances to indicate what sentence he would pass." (My emphasis, quoting then-Lord Brown sitting as a Lord of Appeal in Ordinary, prior to becoming one of the first Justices of the Supreme Court in 2009.)

Officially, we don't allow plea bargains in the UK. We regard such things with great suspicion.
On the contrary, it is officially recognised by the highest courts in the land as "accepted practice", as I quote above. I know it's a common belief that plea bargaining is not done here, but the belief is misplaced. As you note, there are indeed provisions for those assisting the authorities as well - it may be distasteful, but when dealing with drug gangs and terrorist groups, you either need to accept dealing with unsavoury characters, or give up on enforcing the law against such groups entirely - and of course our system as a whole adopted the former approach long ago.