Using A4 pouches as above is probably cheapest but it can be a nuisance getting all the bits laid out and keeping them from moving as you lift it into the laminator.
And then you have to trim them individually as they don't usually line up, so you can't just use a guillotine.
This advert shows the individual pouch sizes available -
LAMINATINGPOUCHES.CO.UK - FOR THE BEST DEALS ONLINE IN POUCH LAMINATE SUPPLIES
Much easier to adjust the size of your label to suit the pouches than the other way round.
Takes a while for the laminator to heat up so it is best to do them in large batches.
I use a cheap laminator from LIDL and bought all my supplies through the 'net - no problems.