I have the Canon Pixma IP90 colour inkjet, c. £180 - excellent for this type of application; occassional printing. You wouldn't want it as your home printer, due to the tiny cartridges and consequent cost per page.
There is the
Pentax Pocketjet which is B&W and which I have seen the output of, and the 300dpi version looks excellent for approach plates. Uses special paper but not the super expensive printer used by the dye sub photo printers. This printer costs a fair bit too, about £400.
Given that one has to carry a laptop anyway, a charger for it, various other bits, I don't consider the IP90 a problem. It's very lightweight. I believe it's available with bluetooth (fairly pointless since a cable is so simple and far more likely to work) and an external battery pack. I didn't buy the battery pack since in any situation where I wanted to print plates I also had a mains supply for the laptop.
There is very little else out there, as far as I can see, which will product big enough paper. There are several other printers, Canon I believe, which look very nice but are only A7.