In a 3v3, I play SU (usually Mechanized Support) my mate plays USF (always Tactical Support) and our third teammate plays Brits. Which commanders should the UKF have in it's loadout and which one should he choose in game and why? (he usually puts down a mortar bunker early and aims for several comets in the late game, the rest is flexible)
2 Regiments can help your Brit mate out in this case:
1.) Commando Regiment: He should choose this Regiment if you have problems dealing with infantry or fortified positions (OKW trucks + PaK43 + ...). Commandos sweep every kind of Axis infantry off when they manage to shorten the distance between them and their objective. If you face huge blobs then it will prove adjuvant to place demos with Commandos at important locations like your cutoff, VP, fuel point and so on. In case your foes choose to heavily fortify your British friend will be able to call upon the services of 2 line breaking abilities: Mortar Cover and Air Supremacy Operation. Mortar Cover will bombard hostile team weapons with white phosphorous shells which are going to drain life points of those. Once hostile weapon teams have almost no life points left you and your mates should push in to decrew everything you can find. If you encounter PaK43 + OKW trucks or similiar positions your Brit mate should use Air Supremacy Operation to flatten the area for his Comets and remaining armour.
His infantry can profit from Commando Regiment either via using "Assault" (4CP) causing his Infantry Sections to move more quickly and fight more efficiently.
2.) Tactical Support Regiment: This Regiment should be chosen if you cannot handle Axis armour properly since it allows your Brit mate to combine his Comets with one of the strongest off map abilities in the game: Artillery Cover. You and your mates should at least amass 3-4 various medium tanks per player (ISU-152 + 3 T34/76s, 2-3 Jacksons (and 2 Calliopes, of course not pushing), 3 Comets for example) being cautious (which means that you shouldn't push outright into hostile territory but nevertheless exert pressure on foes without the risk of losing armour) until your Brit mate has unlocked Artillery Cover (10CPs). If you and your allies are not determined when exactly to push in your Brit mate can solve this problem by designating his Universal Carrier as Command Vehicle thus being able to call in free aerial reconnaissance.
Make use of free reconnaissance, spot weak points of hostile defence, position your armour accordingly and then activate Artillery Cover where your foes gathered their armour. Push in and "Mark Target" the hostile key tank (Elephant Tank Destroyer, Jagdtiger, etc.) to destroy it swiftly while your USF mate should barrage every congregation of support weapon teams with his Calliopes. Once you have destroyed the key targets everyone of you can continue to pick up Axis armour since Artillery Cover paralyzes every hostile tank in the area of effect while pinning down infantry either.