A couple of things... we installed 3 1/2 inch wide oak flooring (actually called 'plankin' due to its width) here in the U.S. about 3 years ago. Full oak about 3/4 inch thick ... really liked it and still do.
One thing, the underalyment is important to avoid squeaky floors. The boards are nailed down with a special air gun and the nails go into the grooves of the tongue and groove part of the boards. The nails (or wire brads) need something really sturdy to be driven into. Best thing is 1/4 inch or 1/2 inch plywood, but many homes, at least here, were fitted with OSB (oriented strand board or commonly called "chip' board) back when it was popular and less expensive than plywood.
Well, we, fortunately, replaced the 'chip' board nearly everywhere before laying the hardwood, with the exception of a difficult to reach area in an entrance way. Guess where the floor squeaks? That's right... So, lesson learned, make sure the subfloor is sound and tight to insure a non-squeak installation. One addendum... here it's recommended a special paper be used between the flooring and the subfloor... it's called Red Resin and is indeed reddish. It's used to further reduce or eliminate floor squeaks caused by friction between the wood layers.
Some installers use the common roofing asphalt 'tar paper' (as it's called here... felt roofing paper elsewhere) as a vapor barrier but I dislike the smell of it in the house...
Good luck!