To reduce the cost of your flight from Helsinki to Inverness, you might
consider purchasing a roundtrip ticket Helsinki-Inverness and not using
the return portion. That would put the cost down to around USD475 or so.
The airlines don't like this strategy (seats go out empty on those fake
returns) but when there is such a huge difference between the roundtrip
& oneway rates, of course travelers will try to get around those
expensive fares! The roundtrip ticket will be much more restrictive in
terms of numbers of seats available, how far in advance you must
purchase and undoubtedly carry penalties (sometimes equal to the price
of the ticket) to change or cancel!
You could also buy a roundtrip Helsinki-London for about USD370 and
train the one-way to Inverness (it's a loooong day's ride), and not use
the return portion London-Helsinki. The same restrictions that apply to
the Inverness option would apply to this ticket too.
Airline pricing structure is rather odd at best.