Dec, 42, was seen flashing his boyish smile while stepping out in the capital.
The Saturday Night Takeaway star looked smart in an all-black suit as he carried his belongings in a checked backpack.
Dec's appearance comes after Ali, 40, was spotted for the first time since their big news too.
The blonde, who used to be Ant and Dec's manager, was seen sporting a hint of a smile as she wore jeans, a jumper and a chic pink coat.
Despite the bun in the oven, Ali didn't appear to have a bump yet.
Dec announced the couple's big news through the Ant and Dec Twitter acount on Sunday evening after rumours had circulated.
He told the TV pair's 6.82 million Twitter followers: 'Just wanted to say thank you for all the lovely messages.
'The news has sneaked out a little earlier than we had hoped but Ali and I are delighted to be expecting our first child. Thanks for all the love, we really appreciate it.'
He posted this alongside a snap of the pair of their wedding day, where they were seen holding hands.
Fans were instantly began sending the star and his wife well wishes on their baby news.
Dec has got a big weekend ahead of him though, as he is presenting ITV's Saturday Night Takeaway solo for the first time.
He had vowed before to never present without Ant, however later said he 'owed it' to Takeaway fans to finished the series.
It's claimed execs of the show are 'pulling out all the stops' to make sure the last two episodes without Ant are a success.