The UC certainly needs something, being able to transport a Vickers would be nice ( I don't mean fire out of it, merely transport it as per it's primary role!). Suppressive fire doesn't seem to be any use, maybe armour piercing rounds so they could nibble at the Axis lights? Not being able to repair them until T2 is the biggest handicap.
Other options which follow a historical path are 2inch mortar upgrade and Boys AT which it pretty much carried as standard ( because no bugger would carry it!).
Maybe a suppressive fire ability for a nominal munitions cost on the Vickers at Vet1? I love the Vickers but seeing a few Axis squads waltz past them accepting a few casualties feels wrong when you've taken the trouble to trench and dedicate a unit to choke point denial. Lobbing a nade and forcing you out is fine, some pre-emptive counterplay, maybe only when not garrisoned in a building, would help a lot.
As for the base arty I think the primary thing it lacks doctinally is smoke ( other than obvious things like accuracy, tot, weight of fire). You get a smokescreen late game with the Churchill, or with a pit at vet1. It is rather shortranged given the vulnerability of a pit on the frontlines though. Being able to fire a smoke mission within los would give the AEC with it's excellent sight range a useful recon and scouting role. Also the primary role of the 2inch mortar was to provide useful smoke at range, few platoons would go out without a couple of them.
Come to think of it giving the UC the ability to call in base arty a la sniper or upgraded IS would give them a role in the late game and wouldn't affect a thing until T2. Would give the Axis lights something to do other than mowing down my infantry down for fun as UC's would need to be hunted.
Other than mines it is very difficult to ambush the Axis on COH2. Piats are too inaccurate against anything moving above tortoise pace and nothing other than the Cromwell really has the mobility. Carden Lloyds were often used to truck 2 and 6 pdrs around. Maybe consider allowing the UC to hitch AT guns ( can't see it, would need graphical changes). They'd be pretty much defenceless to AT on the move but being able to reposition your AT rapidly on a flank would make a world of difference.
British doctrine then and now emphasised recon, particularly for indirect fires, for light armoured vehicles. Stay out of sight, stay alive and call in the arty. Oh and mow down any trucks you happen to see.

The main armament was there to suppress whilst reverse gear and full throttle were selected which makes the AEC a strange mishmash of units. Very decent light AT with woeful AI and terrible handling. Not helped by defensive smoke which only fires after the inevitable engine damage.
The humble Humber would be a better choice I think, lighter armour but with a 15mm Besa which would suppress at great range and annoy the hell out of anything it was too weak to penetrate.
I'm not a good player so take this as more of a historical critique if you disagree.
