What’s a good alternative to rice wine vinegar?

Rice wine vinegar and rice vinegar are the same thing, so that's easy.

For additional pantry-stocking:

Seasoned rice vinegar is just rice vinegar with a little salt & sugar added. It's intended to be the right mixture for a number of recipes, not the least of which is sushi rice, although in my experience you're better off seasoning plain rice vinegar to the correct balance.

Rice wine is, of course, sake. Generally if you're cooking with rice wine you want a moderately priced, clear filtered sake like Gekkeikan. The same advice about cooking with regular wine applies here; don't try to cook with anything undrinkable.

Mirin is a specific kind of light, sweet rice wine used in braised dishes, dipping sauces, steamed fish, etc. If you can't find it, buy a drinkable sake and add around two teaspoons of sugar per 1/4 cup of sake.

First off, I suspect that 'rice wine vinegar' and 'rice vinegar' are the same thing. Secondly, rice wine vinegar comes in two basic kinds, there is a dark vinegar and a white (yellow) vinegar. The dark vinegar has more flavour than the white version. However, both are relatively low in flavour compared to other vinegar alternatives.

To replace a white rice wine vinegar I would suggest using another low flavour vinegar. One with a white colour. In the UK there is a kind of clear vinegar usually labeled as 'non-brewed condiment' which should make an adequate substitute though I'd suggest using a smaller than normal quantity as it is stronger than most other vinegars. Another alternative would be a clear malt vinegar which will be close though not absolutely the same. Again you may need to reduce the quantity to match the strength.

To replace the dark rice wine vinegar is more tricky. My first suggestion is to try a dark malt vinegar as a replacement.

If you want to try to source the real thing, you might not find it in some UK supermarkets. However, almost every large city in the UK has a China town which will contain a Chinese supermarket where you can stock up on any items you might need. Failing that, try asking at your local Chinese restaurant or take-a-way. They might be able to point you to a supplier. Lastly there are online suppliers in the UK if you google you should be able to find one.

rice wine vinegar and rice vinegar are NOT the same. They taste totally different because Rice Wine Vinegar is made from SAKE, Rice Vinegar is not.