Why do the British use rifled barrels? From wikipedia:
"Uniquely among NATO main battle tank armament, the L30A1 is rifled, because the British Army continues to place a premium on the use of high explosive squash head (HESH) rounds in addition to APFSDS armour-piercing rounds. HESH rounds have a longer range (up to 8 kilometres / 5 miles) than APFSDS, and are more effective against buildings and thin-skinned vehicles."
Now having rifled barrel's isn't ideal for APFSDS as it's not required for a fin stabilised projectile and means you have to use slip rings to prevent rifling. Also rifled barrels wear quicker. The APFSDS is the more obvious choice for an anti tank round due to it's high effectiveness on armour, but does not (or at least did not) hold the accuracy or range of the HESH. However, perhaps in anticipation of fighting a holding action against soviet invasion the British wanted weapons which could keep the numerically superior soviets at arms length, that's just speculation.
There were plans to update to the smoothbore Rheinmetall L55 for the Challanger 2, but these were shelved for budgetary reasons.
As far a suspension goes, here is a link. It's from a games forum, but the guy knows his suspensions and correctly points out that different bogies are different. more here
If you were wondering why the British (and germans) still use diesel engines rather than gas turbines like the Abrams that's because the turbines poorer of range and reliability.
Maybe the combination of these, although individually justifiable, does suggest conservatism. But I would point out that it was the Chieftan tank that introduced the supine driver position, enabling shorter and more sloped hulls. Also I can't find any evidence of the British dragging their feet on targeting systems, with both introducing laser ranging finding in the eary 70s, on the UK's Chieftan and USA's Sheridan respectively.