I don't have an IR, but can I file an IFR flightplan and still fly in VMC?
I can't answer your question fully, but I know that the UK is the odd duck in here with regards to night flying. In the UK, a night flight (except in a control zone or something like that) is conducted under IFR, but an IFR flight is legally allowed for a pilot without IR, as long as he/she maintains VMC.
In most of the rest of the world, VFR at night is just that: VFR. There are, as said, specific regulations with regards to altitude, routings and such which should all be detailed in the relevant AIP, but the base set of rules is VFR, not IFR. IFR is only legally allowed if you have an IR.
So if you make an international flight, and you file a flight plan, it would have to be a VFR plan for the portion up to the FIR boundary, and an IFR plan for the UK part. (I always forget whether that's a 'Y' or 'Z' plan.)
The Netherlands doesn't allow VFR at night whatsoever, except police, SAR helicopters and the like. So all flying at night takes place under IFR, and requires an IR.
(The latter is really awkward since the IR requires a night qualification as a prerequisite. But you cannot legally fly solo at night until you have that IR, so you can never obtain the required five solo landings for the NQ. The IVW has solved that with some creative bending of the rules...)
Oh, and it doesn't matter how many engines you have, BTW. Zero, one or more, the rules are the same.