I don't think conventional diesel power is actually a realistic option in practice, given that allegedly none of the ROSCOs or major train manufacturers are presently interested in ordering/building new diesel trains to the UK mainline gauge, despite the current shortage, because of lack of guaranteed future use. The only other UK railway that operates with a gauge comparable to that of the Island Line at present is London Underground and they're never going to be interested in either diesel or second-hand trains (let alone a small fleet of them). While some of the same considerations would also apply to EMUs, a lot of the design work for tube-sized EMUs has already been done and any new work would have potential benefits when LU order their next fleet - which is simply not the case with DMUs.