Similar thing to what I have done with my house (ask Tiff lol)

But my wall wasn't a stud it was solid brick 3.8m long, one brick thick to eaves with timber joists running through the wall spanning 4800mm (4.8m).

I knocked the whole thing down and put a 203 by 102 23mm thick I section beam 3200mm long ontop of the bits I left to take the bounce out of the joists.

Nowt wrong with it, and what I have done is over kill.

Cost me £89 delivered for the steel.

With regards to yours there is nowt wrong with using wood - you said it yourself, your house (as are most in the uk) is timber framed.

The only problem you potentially face is what the timbers he's putting in are supported on.

I have some pics if you're interested but I wont post them for the sake of it as I almost cant be arsed