Best cordless vacuum cleaner for low and high cleaning for someone with a bad back?

I am looking for the lightest, efficient cordless vacuum cleaner in the Dyson range.

I live in a static caravan, largely carpeted (very short pile), but with several laminated hard sections (eg, kitchen/dining area). There are also many curtained windows, and high places (eg, cupboard tops and lights) that act as dust magnets. Oh, and not forgetting furniture (eg, settee and beds) and my car. I envisage using the VC both with a wand and without in about equal measures.
 

Hair isn’t much of an issue, as I’m not owned by a dog or a cat, and I haven’t got too much left to lose on my own head. 

I also have a bad back and weight (or lack thereof) is *very* important to me.  However, I’m willing to trade weight against efficiency and reachability (eg, to vacuum a mattress or cupboard top). And I’d rather keep wand/tool changes to a minimum (eg, when transitioning from carpet to hard floor). 

One question I have is “What parts of a cordless vacuum cleaner are included in its “weight” specification?”  I presume it’s just the bare handheld part?

The Dyson V12 looks like the best choice, but despite what the manual says, there doesn’t appear to be a UK-available up-top tool? Are there any tools that will do an equivalent job and are available in the UK? 

Any guidance gratefully accepted. Thank you.