Do you have a replay? It's much easier to give strategic advice if we can see what you're doing.
they greet my squad with the axis engineer squad, they completely rip me a new one and i have to fall back.
That shouldn't be happening against an Infantry Section if you're sticking to cover. If you're moving all the time and staying out of cover I can see how the Pioneers might take the advantage.
A few useful things to know (apologies if I'm telling you things you already know):
- All units are less accurate on the move. Close range units with fast firing weapons have smaller movement penalties.
- Cover is incredibly powerful. Light cover halves your "received accuracy" which is the likelihood of shots to hit you: the enemy will miss more. Heavy cover gives this bonus and halves the damage you take.
- UKF infantry get cover bonuses on top of this but they're also weaker out of cover than other faction units.
You say you can flank MGs which means you've got most of the solution down already. MGs in buildings can be difficult to deal with. What you need is a way to force them out.
In most factions the answer to this is indirect fire. The other four factions get a mobile mortar with a smoke barrage to blind MGs so you can flank them and a High Explosive barrage to kill MGs that refuse to move. UKF's mortar is a very expensive static emplacement. If you can get this up and defend it you can drive HMGs out of every building in barrage range. You don't always have that option though.
The most effective way to clear a building is fire. Unless you have the Mobile Assault Regiment commander which allows Sappers to equip flamethrowers your flamethrower is on the Universal Carrier. If you aren't good with light vehicles this can be a difficult unit to use but if you can get the hang of light vehicle micro this'll burn out those HMGs with little trouble provided you stay out of their firing arc.
Another excellent option against HMGs but a micro-heavy one is the Sniper. This unit can attack HMGs from outside their range. You've got to be careful with your sniper because if the enemy can close on it it'll die but if you can learn to use this unit well you can pick off HMG models with impunity. An HMG spammer will have limited options to fight back against this unit because they've spent all their manpower on barely mobile HMGs and if they're sticking to buildings they physically can't close in. The trick to using a sniper is to attack move it so that it always fires at maximum range and to fall back if any enemy unit tries to get near it.
You can exploit flanks on garrisoned HMGs with your upgrades. UKF can unlock infantry grenades fairly cheaply: chuck one of these at a building and that HMG's going to get hurt. Chuck two at it and that HMG's dead. You could also give one of your Infantry sections Howitzer flares. Throw one of those at a building and your base howitzers will bombard it. It's 45 munitions but the HMG's got to leave that building or die.
Also remember that HMGs can't garrison buildings you control. Take advantage of that early on: if there's a building covering your fuel that the enemy HMG always takes then take it yourself with your starting squad. Build an HMG as your first unit, stick it in the building your Infantry Section is holding and then your Infantry section can cap points while your HMG covers them.
Finally, consider building a couple of HMGs of your own. HMG spammers usually have poor micro of their own and will struggle against their own tactics.