No, this will not work.
Buying a return ticket and only using the first leg of it will certainly work - just make sure that you don't skip any of the legs before the one(s) you intend to use to avoid the ticket being canceled.
However what will not work is trying to book a refundable fare class for the return, and then obtaining a refund for that leg. The reason is, as you've guessed it may be, in the "fine print" - or specifically in the fare rules.
All airlines will have something like this in their fare rules and/or Contract or Carriage (CoC) :
OPEN JAWS/ROUND TRIPS/CIRCLE TRIPS NOTE -
WHEN COMBINED WITH HALF ROUND TRIP FARES TO FORM
ROUND/CIRCLE/OPEN JAW TRIPS **THE MOST RESTRICTIVE
CONDITIONS APPLY.** THESE INCLUDE MINIMUM/MAXIMUM
STAY /RESERVATION/TICKETING REQUIREMENTS/STOPOVER/
COMBINATION PROVISIONS AND **CANCELLATION CHARGES.**
Specifically that example is from American Airlines, but the others will be similar.
This means that if only one of the flights on a ticket isn't refundable, then neither are any of the other flights booked on the same ticket - even if they are different fare codes that would normally allow for refunds.
If you were to ask the airline to cancel the return leg then they will normally "reprice" the entire ticket, which basically means that they will calculate the price again for the whole ticket - including legs you've already flow! This will put you back into the original situation of being after a one-way, and as the price of that will be higher than you've already paid, there won't be a refund due.
It's worth keeping in mind that whilst one-way flights will sometimes be more expensive, they often come with better conditions (eg, allowing changes and/or refunds) that the cheaper return flights do not. Booking a return and simply not using the return portion may make sense if you saves you money - but make sure that you're not taking a higher risk to save a small amount of money. In this case it looks like you're saving around £200 so it probably is worth doing, but if you were only saving £20 then you'd almost certainly be better off paying a little more for the one-way and the additional flexibility it probably gives you.