Is it bad that I was immediately jarred by the fact it doesn't use Transport Sans? Of course that is actually correct according to the GDS rules, because it's not on a service.gov.uk subdomain.
It's as much a licensing issue as anything I think. GOV.UK uses 'GDS Transport', which is a custom cut of New Transport, a commercially-available font [0] which is available for the low, low price of £10k per weight for unlimited impressions. I've never seen any enforcement though (doesn't mean it doesn't happen) and it's pretty common for services not on *.service.gov.uk to end up using it anyway