I believe I even posted this in the UKF Alpha forums about a while back but nobody paid attention.
My wet dream was to have a sort of Eastern Front Ostheer's gameplay mechanic of switching between Offense and Defense, in the case of the Brits, more static and more mobile gameplay, ever since the Alpha.
Basically my idea was to have Anvil and Hammer be 2 swichable options in the Brit HQ and lock all mobile units in Hammer while locking all emplacements and the Churchill in Anvil, so if you go Anvil you'd get access to the Bofors, 17 pounder emplacement, upgraded trenches (I would just change the model and allow it to garrison mortars, for example) and the mortar pit which like Svanh said would be turned into a defensive position and you would have access to a mobile 3-inch mortar team by default, plus the Churchill and I was hoping that Anvil would also unlock a hull down ability for British tanks but sadly I think lack of animations are preventing that from happening.
While if you choose Hammer you'd get the Universal Carrier, AEC Mk III Armored Car, Comet and Firefly as well as some other offensive ability like the vehicle tracking for example or something.
As for the standard toolset you'd still have the Infantry Section, Vickers, 3-inch mortar team like I already mentioned, Sappers, 6 pounder AT gun, Sniper (altho he would have a Scoped Lee-Enfield by default and be able to upgrade to a boys .55 AT rifle like it was originally intended to be) and Cromwell without the need to invest resources into specializing into either defense or offense, you'd just still get sandbags and trenches by default to hold ground but not really dig in as effectively as if you were to go Anvil or take ground as effectively as you would if you went Hammer.
But yeah, you'd be able to switch at any time but of course would require to invest resources and time again, so researching would need 30 seconds to complete while the cooldown would be like 60 or so before you can switch again, giving the ability to the British Army to adapt to the Battlefield situation instead of locking units and abilities behind senseless side-tech. The only choice that you make that should be locked in is the Commander you choose, not side tech if you ask me, the trade offs are just not worth it.
And yeah sorry for the wall of text but I had to rewrite it and I'm in a bit of a hurry.
This makes too many sense => Lelic will never implement it.