As you are in the UK chances are you'll be given miles till touchdown. If you're given 25nm, for example, multiply that by 3 would equal 75 (7500ft). A rule of thumb is the -800 will lose 10kts in 1nm of level flight. And 1nm x3 equals to 300ft. So at 220kts you'll need to lose 20kts for up speed. 2x3 = 600ft. So 25nm to go at 6900ft would be spot on.
But the important part is that you need to be descending at the right rate (half ground speed). 250kts ground speed you'd want to descend at ~12/1300ft/min. If you're not then you're high or will be high at some point. From then on just estimate how many miles you have left. Easy way to do that is look at the progress page to see your miles left on the as soon as ATC give you your miles and using the difference you can work out your miles left at any point. Obviously the magenta line as to be in the same general direction as your HDG for this to work. So be careful with this one.
Alternatively use the green banana, remember you'll still need to decelerate so it's a good idea to get the green banana to be level early.
Unfortunately it comes with experience on what to do. Flaps are your friend if you're high. If you're not sure how it's going you can always turn to the other guy and ask.