Can anyone advise if this is do-able/worth calling in a structural
engineer for/or is it a complete no-no?

I have a close coupled hipped roof of conventional rafter/joist/binder
design which I would like to convert (and maximise) the storage space
in. My Rafters are 125x47 C16 timber and ceiling joists are 177x47 C16
timber. In one direction the rafters are tied to the joists - at right
angles to that there are 3 binders (ties) sitting on top of the joists
which are 125x47. WHat I would like to do is to tie the rafters (ie
the ones at 90 degrees to the joists) with Galvanised steel OR 47x47
timber and remove the wood binders/ties to free up some headroom. Is
this common practice? can anyone recommend any
sizes/products/supplies/tables of sizes etc?

thanks

Ken