With the 'anvil' upgrade Royal Engineer:
+ free lmg and still have 2 weapon slot left.
+ vet3 they reinforce for 13 MP and with 3 lmgs they vet pretty fast.
+ fast repair.
+ Eng Piats have higher ROF out of cover than IS out of cover.
Against Werh build order should be flexible but building more than one IS will leave UKF to weak against snipers.
IS (starting unit)
IS
1-2 MGs
T2
1-2 Eng
Sniper (unit ability will do eng damage on scout cars)
Spam more Engs before going T3 asap when fuel allow to get the free lmg upgrade on engs (with anvil)
Get 1 brens upgrade on all IS and Eng except maybe one with double Piats that are used for repair/sniper bait. After that get second brens on all Engs/IS.
Also mid game its useful to get a command vehicle (2 CP ability) on a carrier for the free recon plane to know where the sniper is moving. The command vehicle also give a huge buff to Armor reload and some minor buff to infantry/emplacements.
If floating manpower mid game/late game get either caches or use it to lock down the central VP with a mortar/bofors + FHQ (for the garrison bonus + reinforcement). For 45 munition the FHQ can give artillery cover from the base to the VP area and also buff the mortar/bofors. The bofors have close to the same range as mortar with the artillery ability (garrison bonus).
my 2 cents