As
carnarvon says, it does not have to be in a 25% earning class.
KLM.nl is currently advertising returns to London in Europe Business Class (how posh!!!! Not!!!) for €467*. This is cheaper than 4 Economy returns in your 25% earning cheapest class of €132* and will take up far less of your time.
*both figures as reported by the on-screen low fares calendar, without clicking through to verify and/or inspect dates and availability - see below for the €132 figure.
or if you want to be even more fancy, take Garuda's First Class to Gatwick on this coming Saturday, back on Monday, for €542. You can play with dates to see if cheaper biz/First fares pop out on dates that work for you but remember that GA doesn't fly AMS-LGW-AMS as frequently as they used to (and won't, anymore, beyond March 2016 if I remember the date correctly). Just verify that the class you will be booked into earns the sufficient number of miles for you before committing - some of these "promo" buckets may, given FB's noted generosity, earn you nothing!!!!