giskard

The rear wheel on my Dahon Mu P8 is rubbing against the left brake block, and backing off the brakes a little "cured" the problem, but after riding to work this morning it got worse to the extent that I can now see where the rim is damaged anfd hitting the brake block.

Is it worth getting the wheel repaired? Or am I better off just buying a replacement? Do I need to pair wheels from the same manufacturer?

dabac

A brake block won't hurt a rim any time soon, unless you've worn the block down to where the metal core contacts the rim. Even then it would take some use before serious damage occurred.

And no, wheels don't have to be replaced in pairs.

Assuming there actually is significant rim damage - given the scarcity of 20" wheels built up around gear hubs like that your best option is probably to have the wheel rebuilt with another rim. Luckily the BMX crowd ensures that 20"rims are aplenty, and you might even be able to find a replacement that lets you keep the spokes.(if the wheel builder agrees with that.)

Then get the return spring tension of those brakes balanced before riding again.