Hi Keith,
I use the small plastic palettes (I have several) you've described, but I also use larger ones that have rectangular wells rather than round ones...works fine for me.
Looks like this:
Sometimes I also use old ceramic tea-cup saucers and/or dessert plates if I'm mixing large quantities of color, these also work fine and don't cost a thing, eh?!
If you're working for a long period of time, you can take some cling wrap and cover all, or part, of your palette(s) - helps to keep the paint from drying out too quickly.
I also have another plastic palette that has little clear plastic jars with lids. I put the paint in those (or mixes) and they sit in wells along the sides of the palette... then take out what you need and mix further on the center of the palette. It's called the ColorsTo Go palette and I ordered it from Jerry's Artarama in the US, but maybe you can find something similar in the UK?
Looks like this:
Hope that helps a little.
~ Judy