Mary Reynolds, the poor man's Jeanette Charles as Her Maj in Doctor Who - Silver Nemesis and Michael Winner's Bullseye!
Janet Brown as Thatcher in For Your Eyes Only, TOTP, Mike Yarwood, Election '79, and John Wells as Denis in FYEO, Anyone for Denis?, About Face, a Basil Brush panto.
Julian Glover as Richard Couer De Lion in Dr Who and Ivanhoe. Both Camfield.
Patrick Macnee as bowler-hatted, umbrella-wielding British secret agent "S" in The Hardy Boys. Guy DOleman in Murder, She Wrote as a Chief of British Intelligence - codename Corsair who was either his Number 6 or Colonel Ross from the Harry Palmer films, likely the latter, possibly both.
Roger Moore as James Bond in Mainly Millicent - in 1964.
Huw Edwards appears in both Doctor Who and Skyfall - therefore he is the first actor/character to appear in both franchises, thereby semi-officially linking the two. Sadly, Who has Bond as fiction, but then it also hinted as Who itself being fiction in the world, in Remembrance of the Daleks.
Similarly, Kenneth Kendall played a sort of future version of himself in 2001 - A Space Odyssey and as himself in Doctor Who.
And Gordon Honeycombe links the Professionals, the Fourth Protocol, the Medusa Touch, Castaway, the Gentle Touch and Bullseye! among others (plus Bullseye could be argued as being in the Whoniverse, as Mary Reynolds - Britain's second-most famous Queen lookalike plays the Queen in both Bullseye! and the Who story Silver Nemesis)
Also Jeremy Thompson of Sky has varied credits.
Howard K. Smith played himself in CLOSE ENCOUNTERS OF THE THIRD KIND, THE BEST LITTLE WHOREHOUSE IN TEXAS, THE PURSUIT OF D.B. COOPER, NASHVILLE, THE CANDIDATE, "The Bionic Woman", a deleted scene in the Pink Panther Strikes Again, "V - the miniseries" (1983), and "V - the TV series" (1984-85).
Ralph Bellamy and Don Ameche in Trading Places/Coming to America.
Ken Tobey as his Thing character, Hendry in the Naked Monster.
Dick Miller as Walter Paisley from A Bucket of Blood in various.
And Dan Aykroyd as Ray in Ghostbusters and Casper. Dirk Bogarde turns up as Dr Simon Sparrow in 'We joined the navy', Bruce Willis as John McClane appears in 'Loaded Weapon 1' and Cushing and Lee as Frankenstein and Dracula make an appearance in 'One more Time'.
John Laurie played Ebenezer Balfour in both the 1956 BBC version of Kidnapped and the 1960 Disney film version.
Charles Gray as Mycroft in The Seven Per Cent Solution.
Buddy Ebsen cameos in The Beverley Hillbillies movie - as Barnaby Jones.
Also, I think the Sharif assassin in Pink Panther Strikes Again/Tamarind Seed character is Agent Cedric in Top Secret!
Jim Mckrell as news reporter 'Lew Landers' appears in both The Howling and Gremlins.
Patrick Macnee was Dr Watson to both Roger Moore and Christopher Lee (who played two different versions of Holmes)
Peter Cushing played three different versions of Sherlock Holmes (TV series, film, TV Movie)
Kevin McCarthy turns up as his Invasion of the Bodysnatchers character Bennell in the original, the remake and 'Looney Tunes - Back in Action'
And Douglas Wilmer reprised Sherlock in"The Adventure Of Sherlock Holmes's Smarter Brother".
Christopher Plummer also played Holmes in Silver Blaze (1977) before he did Murder By Decree (with Frank Finlay reprising his role of Lestrade from A Study in Terror and Hilary Sesta playing the same prostitute she did in Barlow and Watt - Jack the Ripper). In Silver Blaze his Watson was played by Thorley Walters, who also played Watson in Smarter Brother, The Deadly Necklace and the Best House in London!
Hattie Jacques as Matron in the Carry Ons, though none of them are in the same continuity, apart from Carry On Doctor and Camping. She claims that she was working in a hospital in her last job and the doctor looked just like Kenneth Williams' headmaster...
Olive crossing from On the Buses to Rag Trade. But not quite in canon.