I’ve had the tyre repair in aerosol form for too long and I want to get an actual spare wheel. 
if I get a 16” steel wheel with a 16/55/205 tyre on, will it fit in the boot under the floor or must I get a space saver version? I know what I’d prefer but I need wisdom. Please help!

A 205/55 R16 will be fine as a spare wheel. 

Yes i did exactly that, took that exact spare tyre out of an 11 plate octy mk2 and fitted perfectly in my mk3 octy space in boot... On trade in.