With separate units, hob on a work top, oven built under if one goes pop the other still works, independently, on site if it needs sending off somewhere.
However, if you have a free standing already, you'll need to replace the work top to change from one to the other.
Built under, are limited to size of the oven spaces due to the height of the work top down the level of the plinth. So you will get a bigger space to fit ovens in with a free standing.
Built in are different again, as they are at built in to a floor to a ceiling type unit, so you get spce to have two big ovens in a built in.
Thing is your stuck with the design of the kitchen from the outset. So major works is required to change styles.
Re our current predicament, best price we have is £450 for a belling fse60dpss, at Kevs suggested place in Tadley.