It's generally possible to fix a rack using p-clips. And if you are only planning light touring you wouldn't need a rack for the front. Alternatively, dispense with the racks altogether and tour with a carradice or other capacious saddlebag and a bar bag. As you say, if it was built for 27" wheels clearances aren't going to be a problem. The only thing to be careful of is making sure the brakes have enough reach to operate satisfactorily with 700c wheels.

Seriously, that Mercian is a classic. I'd certainly be riding it if it were mine.