British Prime Minister Margaret Thatcher did a pointless disservice this week to an important dream.

She did so by throwing cold water on the hope that the European Common Market might eventually evolve into a United States of Europe, as envisioned by the Common Market's founders.Rejecting calls for uniform taxes and a central bank for the 12-nation Common Market, Prime Minister Thatcher claimed that hers was merely the voice of reason and practicality when she voiced hopes that a United States of Europe would never come to pass.

What she described as reasonable and practical, however, might better be termed a snub of the Common Market's past achievements and a failure to appreciate its future potential.

Just look at how far the Common Market has come since it was created 31 years ago. Among other milestones, it has:

- Eliminated custom duties for goods that cross European borders.

- Developed a common European currency which, though used mainly as an accounting device, is becoming more popular in consumer transactions.

- Created a common European passport.

- Jointly developed many major weapons systems.

- Produced a growing degree of political unity. Since 1979, members of the European Parliament have been directly elected by citizens of the Common Market nations.

What's still lacking? Besides a common tax policy and a central bank, the main ingredients still needed to turn the Common Market into a United States of Europe are a common foreign policy and a reduction of the language barrier created by the wide variety of tongues and cultures on the continent.

Though the language barrier may never be entirely overcome, the progress made by the Common Market so far indicates its ability to surmount many of the other problems.

But it will be needlessly hard to surmount those problems as long as Prime Minister Thatcher and many others like her remain obstinately pessimistic.

The need for greater European unity should be clear. As long as Europe remains fragmented, it remains vulnerable to the internal rifts that produced two world wars - and to a communist challenge that is becoming more sophisticated.

The blueprint for a United States of Europe should be seen, then, not as a wildly ambitious dream but as a reasonable response to a serious challenge. It may not be achieved until well into the next century, if then. But Europe should keep working at it.