Mickoo... if a model is to use Slater's wheels then the relevant product for O/F engines is a press fit on the axle with the back-to-back set by shoulders on the axle. Wheels are pressed onto the axle, up against the shoulder, with the fit of wheel on axle achieved by a taper on the axle and in the bore through the brass centre of the wheel. Whilst this topic started here in response to a number of WTers building MMP Cl.08 kits the problem of providing axles for other S7 models of O/F prototypes has been with us for many years, for example GWR Cities and Bulldogs, so resolving the problem of suitable S7 axles is a problem which some of us "kettle" lovers are keen to see happen.
I have discussed, with David White, boring the brass bush in Slater's wheels so as to convert from a taper to a parallel bore, as you have suggested... David convinced me that such an approach was not a good thing as there is a significant probability that the process could distort the concentricity of the bore and tyre.
regards, Graham