Is this a temporary offer, or part of a drive to eventually clear inventory? Probably the latter, I'd imagine that sales of the IDOL 4 Pro might only continue until the summer of 2018. But while £420 was on the high side for the specs in 2018, I think that £320 is quite good value, especially considering that it's still the highest specced Windows 10 Mobile smartphone, arguably.
You can find the IDOL 4 Pro at that price here on Microsoft's UK web site.
PS. See also my original review verdict on this smartphone here.