It does seem very low, however I had made this point in another thread previously and will make it here again:

You booked GLA-DXB-AKL-DXB-GLA with a total distance travelled of approx 25,000nm (24,914nm according to gcmap.com).

You have been downgraded on the GLA-DXB segment, which constitutes 3,633nm, thus 14.6% of your overall trip.

Even if they gave you this segment for completely free, you'd be entitled to 14.6% of your fare as a refund, which is 875GBP. You are, though, still travelling in J, so you could consider that your cost for that sector is approx. 550GBP.

Another thing to keep in mind is that the departure tax in the UK does not change between J and F, the refund would be due to change in base fare and carrier-imposed surcharges.

If you look at the above numbers, you're paying about 63% of the F price for J, (i.e. you're being refunded 38% of your fare paid for that portion of the journey (329GBP/875GBP, 875GBP being 14% of 6k GBP, 14% being the pro-rata distance of GLA-DXB within your overall routing).

It's frustrating, because the gut feel is that it should be more, in fact I was about to agree with Saltire74 about the Flex Plus recalc, but looking at these numbers I think EK isn't being totally unreasonable, though certainly not generous.