what is the V symbol on NATO us UK tanks ?

The V or ^ o r < or > are part of an identification system for tanks. The symbol helps to identify friendly tanks. The different placement is used to identify different units.

In fact, it is the Lambda symbol, as others have already stated. It is a a Visual ID, painted on the vehicles for recognition.

Victory. Or Article V of the NATO treaty = solidarity:

Article 5

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security .

Visual ID. It lets the company commander know which platoon the tank is it.

I think it's to identify friendly tanks.

It is the Lambda

λ