What is the difference between ranch dressing and ranch mayonnaise?
I don't know whether the "ranch mayonnaise" you had was a commercial product or simply something the supermarket whipped up to put on sandwiches.  I personally had never heard of "ranch mayonnaise" as a supermarket product, but an internet search tells me that apparently some companies do market it.

In any case, both the dressing and mayonnaise are likely to contain a similar mixture of herbs and spices (which makes them "ranch").  The difference is likely to be texture.  "Dressings" tend to be more runny and thin, so they can be mixed easily with other things even at the table (e.g., salads).  I would imagine that "ranch mayonnaise," on the other hand, should be somewhat thicker (like plain mayonnaise), just with the herbs and spices of "ranch dressing" mixed in.

Bottom line is that ranch mayo is probably more spreadable (as one would want on a sandwich) instead of thin and pourable (as one would want on a salad).  One could put ranch dressing on a sandwich and get a similar flavor, but it might be somewhat more messy to eat.  Usually the dressing is based on some combination of buttermilk, yogurt, or sour cream mixed with mayonnaise to give it a thinner consistency.

I don't know where to advise you to find the product you want, but you could likely approximate the flavor by adding some of the herbs and spices of ranch dressing (usually some combination of garlic, onion, mustard, chives, parsley, dill, and black pepper or paprika) to plain mayonnaise.  I don't know if these are available in the UK, but US supermarkets sometimes sell a packet of "ranch seasoning mix," which you could use rather than buying all of the ingredients separately.  (If I bought a sandwich from a supermarket that claimed to have "ranch mayonnaise," I would assume that's probably what they did to make it.)  

Or, for a simplest solution if the spice mix is unavailable, you could probably mix a bit of ranch dressing into plain mayonnaise so that it's still thick enough for a sandwich (though perhaps without as strong of a flavor as adding herbs directly to mayonnaise).