The Viral Knight is not some guy who jumped into a suit of armour. I believe that the Viral Knight was a already possessed suit of armor sent to England by Dracula.

(Putting aside the dev's stance on Transylvania being non-canonical.)

Dracula in the novel is shown to be planning on taking over England in particular.Thus it would make logical sense for him to send over his minions in advance to England as museum pieces for them to awaken and join him when he arrives there to serve as his army.The mueseum is new and thus wouldn't question some anonymous person sending over museum worthy pieces. Also the Westminster Knight is almost identical to the Halberd Knights he summons to kill you in Transylvania. It also would provide a good reason why the Knight has a key on him. Dracula could provide the Knight with it when visiting the Museum to settle the sale with the museum. The Knight, when reawakened, could free itself from the Museum with it.

The knoght reawakens because Transylvania could take place concurrently with Westminster, so when Dracula summons his armoured soldiers the Viral Knight could reawakened as the survivors were in it's room, thus giving an explanation.

(Its just for fun and not too be taken seriously)

The Viral Knight in Westminster is a minion of Dracula.