If you are using the official miniatures from UMW, I would suggest you go for 1:43 vehicles as the UMW figures are slightly bigger than most 28mm figures.
My collection consists of mainly MoY by Matchbox to match my 28's as they tend to be slightly smaller, but they are 'box-sixed' and not a consistent scale. Some of the larger ones may work for you, though.
Lledo Days Gone tend to be a lot smaller and do not always work well with 28's and will be too small for UMW figures, in my opinion.
I have recently been buying 1:43 ertl cars for my own Lemonade Wars figures and they would work well for your intended use.
They are rather nice too.
All picked up on ebay UK fairly cheaply.
There are a few cars from my little collection for SA here, showing the slight differences in scale amongst the MoY that I use here:
http://leadadventureforum.com/index.php?topic=39808.1800More detailed pictures of them next to 'Shotgun Bob' the car showroom Cultist from Pulp figures are dotted around that thread.
Note that the two vehicles at either end of the top row are Days Gone By and are noticeably smaller.