A modern tank's primary anti-tank round is the APFSDS: Armor-Piercing, Fin-Stabilized, Discarding Sabot. It shoots a dart-like projectile that uses a sabot, or break-away surround, to fill the barrel, which breaks away after exiting. The dart, or penetrator, is the stabilized by fins. Spin in this case, is undesirable, as it reduces the effectiveness of the fins. Rifling is unnecessary and actually reduces the exit speed of the round. The other types of rounds (HEAT, etc.) have been modified with fins to compensate for the missing spin, accepting slightly worse accuracy for improved APFSDS performance.

Rifled guns like the Challenger has use an obturator, or slip-ring, around the sabot to prevent the rifling from imparting spin.

Donald E. Carlucci, Sidney S. Jacobson, Ballistics: Theory and Design of Guns and Ammunition

To add on to what others said as well, the primary reason the U.K. still uses rifles guns is due to their preference for HESH/HEP rounds.

Due to the nature of those rounds, the head of the shell is relatively soft metal and the entire round has a lower muzzle velocity and is designed for demo purposes as well as destroying older armored vehicles, anywhere where HE would normally be used but minus the chance of shrapnel hitting your own guys. Other nations like the US and Germany primary fire HEAT-MP, multipurpose HEAT as it functions the same being good for anti armor or anti personnel.

The difference is that the the higher breech pressures and muzzle velocities of smoothbores negatively affect HESH rounds as the shell walls need to be thickened for the round to not misfire in the breech, which reduces the space for explosive filler and thus reducing the power of the rounds. So the UK has been the one superpower to stick with rifled guns.