Heart beat starts at around 5weeks, 3 days, and will be seen on scan by 6weeks, but they might have to use an internal scanner to see it that early (the normal scanner may not pick it up until about 7 weeks, depending on the position of the uterus), a sac, fetalpole and yolk sac should also be visible.
With an early scan at 8 weeks you would see a head/body and limb buds, a beating heart, and they would be able to count the heart rate and measure the crown to rump length of the fetus, and the crown to rump measurment will tell them how fast the heart should be beating, and these two together give an indication of fetal health.
The size of the sac, and the yolk sac, in relation to the size of the embryo is also an indicator of fetal well being, so there's actually a lot of "indicators" they can use to assess fetal viability - the big one being a visible heart beat.
It is the size/heartbeat, sac size, presence of a yolk sac of appropriate size that they are looking for in these private "viability scans". They would also rule out ectopic or molar pregnancy, as these would be evident at this stage too.
It's up to you to decide if it's worth £160, if money is no object, sure, why not.
I've had LOTS of early and repeated early scans in the past couple of years (several m/c and now a very closely monitored pregnancy of 23 weeks), so I have a good idea of what they can see from week 4 1/2 onwards, but I've fortunatly not had to pay for any of them. It could either reasure you, which would be great, or warn you things may not go well - if it was bad news would you rather know that
sooner rather than later? If not, don't spend the money!