You might as well walk it if the weather is good. No more than 20 mins and it's a really nice route through the park and then along the Mall, straight across Trafalgar Sq then you're there.Cheers mate. Is it close to Charing Cross station? I'd probably just jump on the Tube as I'd be a bit clueless with the buses.
Edit. If there's any other good shouts re other stops between St James and Kings Cross I'm happy to jump off there for 90 mins before making my way back.
To get back to KGX from around St James' means walking to Victoria and getting the Victoria line, and there's nothing en route.
You could do a short little crawl in that time around Cov Garden up to Holborn. The Lyric, The Harp, Craft Beer Co., Lowander Belgian cafe. Short bus ride from there to the station.
