Hi - I have fitted a similar wooden flooring product (in my case it was a 3 ply engineered real wood T&G product) to my last house - did Dining Room, Sitting Room and Hallway.
I understand that the engineered solid wood is actually more stable then the solid wood and deals with expansion/contraction better and the top layer is usually good for about 3-5 resandings, which should be enough for most people
5-10 mm height variation is quite a difference if you want to have the flooring sweeping right across - B&Q sell various type of underlay - they have the thin foam roll stuff about 1-2 mm thick, the link you supplied shows an underlay 3mm thin which look quite good and they also sell packs of 7mm thermal underlay sheets which is designed to take care of irregularities as well.
If you were to use say the 3mm underlay throughout and then use the 7mm underlay on the area of the floor that is 5-10mm lower, that would give you a reasonably level overall effect, I would think.
Obviously the other option is to the level the floor.
Generally insulation involves thickness - how much raising the floor is to much raising?
I laid my flooring over existing wooden flooring - the reason I didn't just use the old flooring was the amount of draughts coming up through all the gaps in the floorboards which made the rooms very cold. After laying the new flooring - which was T&G planks glued to each other in a floating floor setup over basic thin foam roll underlay, that made it a lot better BUT we then found we were getting draughts from under the skirting (this was put back slightly higher then the flooring to allow expansion) so using something like a cork strip to allow movement but plugs gaps is a good idea.
Oh - and if you don't have them already, get some knee pads!
I went to a bookstore and asked the saleswoman, "Where's the self-help section?"
She said if she told me, it would defeat the purpose.
