Please, never advise new players/player to Blob
It's both easier and more effective than trying to force them into playing spread out around the map. "Tactical" blobbing has it's uses but new players should realize it has it's consequences (demo/indirect fire/crushing).
OP
I feel as though their infantry(starting unit, they may be their engineers) completely shut mine out so I cannot get aggressive if I try to build a mortar pit instead
You might only win against a rushing Sturmpioneer if your starting Infantry section is on green cover or inside a building with enough windows (3 or more, 2 depends on damage done on approach). This means either you need to go for a 2v1 or at least 2v2 situation. Either go with your partner or wait for your Bren/IS/MG. In general, it's better that your startin IS goes to the front and secures a good position and just let your partners RE to do the backcapping.
and it is quickly rushed down or the enemy mortars take mine out rather quickly since mine is static.
Focus on counter-firing first. OH mortars get shredded by yours and ISG (OKW) only might get more range, when they are vet3 and with the barrage (which is not as effective as direct fire).
If they lay their base down its got the turret on top that suppresses and penetrates armor so there's no getting past that defense. The OKW also back cap better than I can assume dominance.
AT guns with attack ground (when you don't have LoS) at max range can deal with it. Smoke or screening with armor and using the flare to barrage with pounders is also an option.
Doctrinally, you can use offmaps to destroy any truck (but this is more of a late game option). Concentrated artillery (Special weapon) or Air supremacy (commandos) destroy or leaves truck at 10%.
What are the weakness of the OKW and how do I exploit and abuse it? Finally, what is the strength of the British army?
OKW weakness: Early antigarrison (occupy buildings with Vickers/Maxims). Early light vehicle attacks. Lack of non doctrinal mobile snares or suppression. Early/Mid cost effective AI (takes time to start rolling the vet). Vulnerable when Flak HQ get's destroy as they'll lack been able to field tanks (unless Call-in but those are limited).
UKF strenght: in comparison to other allied factions, they have the tools to lock down territory but have hard time making a push early on. Bren carriers help against Kubel openings and your MG micro at the beginning should be priority. Midgame, you can either try to go for a sniper/Aec/mortar to put pressure on your opponent. In some maps, you can try to go full cancer mode with emplacements but take in mind, that it's like a cards castle. Once they break your position, you probably lose the game.
A quick Cromwell (reminder that their best AI comes from crushing) can put a lot of pressure against OKW. Don't bother upgrading nades, get the 5th man upgrade if you go for Brens on your IS. Once you lose IS, it's better to get RE (royal engineers) than fresh new IS.
For really late game, Hammer and Comets is the way to go. If facing heavies, double PIATS RE with attack ground are really effective as they will have a hard time dodging them in comparison to medium.