I'm in the same boat as olyreed: [I think I misrepresented olyreed there.] I want as close to 1/55 as possible, if I can get it. 1/43 looks enormous to me - the vehicles swamp the figures. But 1/64 looks implausibly small - I can't see how anyone 28-32 mm high could fit in the vehicle. Obviously, tastes vary.
For diecasts, some possibilities are:
Siku. 1/55 is one of their standard scales, and they have quite a few modern (21st century) vehicles in particular. They also make lots of lorries and farm machinery in 1/50.
Mattel's Pixar cars models. Nominally 1/55, and usually pretty close, with a couple of exceptions ("Filmore" the VW bus works out at 1/61, and "Tex Dinoco" the Cadillac Coupe de Ville at 1/66). See what Constable Betrand did with the Police Cruiser and "Finn McMissile" the Aston Martin DB5 here:
http://leadadventureforum.com/index.php?topic=57694.msg806608#msg806608
Do look at the pictures of each model before buying: some are much cartoonier than others.
Lledo Days Gone diecasts. They don't produce many vehicles for the Cold War period or later, but they are handy if you want to give an agent a vintage sports car. Their 1930 Bentley 4.5 litre blower, for example, is ideal for John Steed and works out at 1/53.
Welly/Nex Models. The packaging says 1/60, but they are actually made to fit the box. So their VW Beetle (Type 1) works out at 1/57, their Citröen 2CV at 1/51 and their Austin Mini Cooper (the 1960s version) at a whopping 1/46.
Siku and Welly are, I think, still in production. There are lots of Pixar cars in China (so many that I am not certain they are legit). I vaguely remember something about somebody taking over the Lledo range, but my recollection is so hazy that I can't rely on it.
Sloppy Jalopy makes a resin commercial van that looks a lot like a Ford Transit as part of its Cold War 1/56 range, and Crooked Dice also makes a Transit van (and a Ghostbusters-style ambulance - I don't know if it's possible to leave off all the extra kit and use it as a regular ambulance). Akula made Land Rovers and a VW van as part of his Apocalypse isle project, but I don't know if he is still contactable.
Wot 'e sed. Plus the Tomica option and the odd Dinky from the 1960s. With the latter you really need to eyeball the item in question as some are way too big. Their Ford Anglia is a perfect fit for 28mm. Their Bedford R/S model works at around 1/60.
The Welly items are also sold as store generics by a few chain stores for a couple of bucks apiece. Locally K-Mart has carried some, mostly the more modern items but I picked up the 2CV and the Mini. Avoid the temptation of the Austin Princess taxi, it's way too small, albeit a useful source of spare wheels for the others. Not sure whether there's a similar chain option that carries them in the UK.