What class are you looking at?

I checked the LHR-DEN route and for 21 March there is 1 seat in Y and there seem to be seats on Y on other dates either side. Same story for a return in Y around 1 April.

For J there seems to be no availability at all in late March and returning in early April.

Cost wise a Y rewrard return including the domestic legs comes in at 50000 avios and £398.