As a44bigdog pointed out, the UK units begin their turn in a sea zone with an enemy sub, automatically placing them in a combat situation. Their choices are to fight the sub or flee. If Karelia is held by the Axis, the UK transport could move out of sea zone 5, pick up units in Great Britain, and move back into sea zone 5 during combat movement (assuming sea zone 4 is friendly). At that point the UK fleet would attack the sub and conduct an amphibious assault against Karelia.
However, if Karelia is in Allied hands, the situation is different. Since the fleet, including the transport, must either move away during combat movement or fight the sub, the transport will have either made a combat move or conducted combat. This would prohibit it from picking up units in Great Britain and moving them to Karelia during noncombat movement.
Basically, the only way the transport can move units in this situation is during combat movement to make an amphibious assault, and then only if it can move to a friendly sea zone to pick those units up first or if the units were already on the transport at the beginning of the turn.