A centrepiece event TfL's Year of the Bus.

http://www.tfl.gov.uk/campaign/our-events

---quote---
Sunday 22 June - Bus Cavalcade in Regent Street

Around 50 buses, spanning the last century, from Horse Buses through to
the very latest New Routemaster will be on display. Visitors will be
able to explore the buses and take part in a range of family activities.
Free entry
[snip]
---/quote---

Also see...
http://www.ltmuseum.co.uk/whats-on/events-calendar#regentstreet
....and...
http://londonist.com/2014/06/bus-cavalcade-comes-to-traffic-free-regent-street-this-sunday.php


Looks great, alas I'm away this w/e but I look forward to seeing some
good photos of the event.

Also today (Sat 21) the modernist marvel that is Stockwell Bus Garage is
having an open day.

http://www.ianvisits.co.uk/blog/2014/06/16/stockwell-bus-garage-open-day/