A mid-range workstation costs over a grand and it doesn't take much to go higher. Add in the support contract and software and £3,500 is easy to reach.
As I said, £3,500 would be absurd for an average desktop. But it's obvious that in an organisation as big as the UK government there will some users who require more than the average desktop. Do ya think that maybe £3,500 was cherry-picked to get some headlines?
Dell & HP workstations are actually decent quality, in my experience. YMMV.