Why was our garlic butter made in France, much nicer than garlic butter in England?

The UK supermarkets have the opinion that British people like milder garlic, so most of the garlic you find in the UK supermarkets, even organic garlics, are of a very mild variety. Also, they are bred for yield, in softer soils, so they produce bigger bulbs but less strong.

Garlic found in most French and Italian stores is much stronger, as that is what people demand. It is usually hardneck, rather than softneck garlic, and hardnecks tend to be much stronger. French garlic is usually grown in stronger soils, so it produces a stronger flavor.

My solution is to grow my own, 3 bulbs planted usually yields a years' worth of garlic for me, and because I have clay soil it has a strong flavor. It's by far the easiest and most rewarding thing I've ever grown.

If that's not your cup of tea then ask a greengrocer to stock a stronger variety of garlic, or order it from an Isle of Wight supplier. Of course, you could just use about 4 times as much to get more flavor.

The comments taken together are essentially an answer.

Not all butter is the same. As SAJ14SAJ said, French butter is normally made from cultured cream, giving it a different flavor. Additionally, the quality of butter depends on the quality of the cream it's made from. So even different brands in your grocery store at home will be different, some more buttery, some lighter, fresher and creamier. It should be no surprise that you managed to buy something different in Paris.

Not all garlic is the same. This isn't really about French vs British garlic. There are many varieties, some with stronger flavors than others. Different ones are grown in different regions, and even the same one grown in different regions will vary a bit. You may well get slightly different varieties throughout the year even in your regular grocery store. So again, don't be surprised that you got something different in Paris.

If you want to reproduce what you made before, you'll have to experiment. You can try different brands of butter; unfortunately, there's some chance that butter like what you had in Paris will be more expensive and perhaps hard to find. You'll also want to shop around and try more varieties of garlic. You should be able to easily notice differences in flavor from just eating a bit raw. It's hard to give specific advice, though - differences in appearance are pretty subtle, and everyone's stores are different. You may want to try specialty stores or farmer's markets.