What will Mantis do that Reaper isn't doing now?
It may do nothing at all, as this is all still a matter of study contracts, and not production orders.
Whatever it does, it will do it WITHOUT spending dollars, without worsening the balance of payments deficit, and while simultaneously supporting UK industry as it breaks into what may be a vital sector. Every penny spent will generate UK tax revenue, and will support UK jobs.
Of course no military programme should be undertaken in lieu of industrial aid, but if a programme can benefit UK plc as a side effect that's great, surely?
It will do it with twin engines - which may or may not improve the loss rate.
It will do it with a brand new configuration, and not as the stretch of an ageing existing design.