Yes should be fine to refund minus £50 before travel. However, after departure, "Refunds permitted for £373" sounds fairly punitive so presumably means you lose your fare.

Having said that, I'd take a careful look at the fare difference and think of it carefully depending on what you think is your likelihood to need to cancel. Say your ticket cost is 150 eco plus or 400 semi-flex. If you cancel, then yes, you'd get back 350 on the semi flex ticket and only, say, 50 in taxes etc on the eco plus.

However, if you do fly as planned, you will have wasted £250 which makes it an expensive insurance policy if you only had 25% chance of cancelling. Furthermore, if changing your trip was an option, you could have still got away with the £150 ticket and simply changed to different flights/trip possibly for an extra £50 penalty compared to the semi flex which would then make that one the far better insurance policy.