Well, several opinions on this matter (thank you) with most going for the premium brands (not-cheap) options.
However, I am not about to spend loads of money on a cordless vacuum. Neither am I prepared to buy on the basis of general advertising blurb: I'm not interested to buy if they are not prepared to quote some sort of comparator rating (motor Watts, Kpa, AirWatts).
In one comparative review that I found, an Onson brand model (£140 - Amazon offer price £110) knocked a Dyson V8 (£300++) into 5th place for various reasons !! (It was Shark, Gtech & Bosch models that beat the Onson into 4th place though.)
At the mo' I'm tempted by a Hosome brand vacuum with 12Kpa standard suction rating and 20Kpa boost rating (that suggests quite good suction, even in standard mode). However, I've noted that a refurbished Dyson V6 Animal (1 yr warranty) can be got for £150 and Dyson reckon it produces 100 AirWatts, which is v good for a cordless "upright". (Edit: I believe AirW measure is the best way to rate I believe cubic capacity per minute is the best way to rate a vacuum cleaner, although few manufacturers divulge that rating for their machines, including, I believe, Dyson !!)
Not sure which way I will go yet.