PIAT gets a bad wrap for being unwieldy, but it's warheads could penetrate the top and side armour of most German late war tanks, because no one is stupid enough to engage a tank head on even with a Panzerfaust or Panzerschreck, plus it has the added advantage of producing no backblast like the Bazooka or Panzerschreck, which means it can be fired from within a house or bunker without destroying or damaging your cover. If the Army can develop the Burney Gun earlier, basically a recoilless rifle, that might be better. EDIT: I should also add re rifle grenades, that the PIAT was used as an ersatz mortar in the absence of enemy armour, in periods of static fighting such as on the Senio river in northern Italy during the winter of 1944 and spring of 1945.
Bren and Vickers are perfectly fine in their designated roles, you'd just need more Brens or a better sub-machine gun. If the British Army can get the Sten Mark V or Sterling into service sooner, or perhaps develop/adopt the Owen Gun that the Australians used, that should be fine. No. 2 Operational Research unit, attached to 21st Army Group during 1944-45, recommended adding a second Bren Gun to the section, essentially creating two Five-man fireteams centered on a Bren, or keeping the one Bren but rearming all but two rifleman with Sten Guns.
Earlier adoption of the 'Pepperpot barrage' method could improve infantry assaults, if divisional RA can coordinate the fire of Vickers guns, Bofors guns, 6 and 17 pounders, and 3 inch and 4.2 inch mortars to shell the immediate vicinity of position being attacked, along with whatever tank guns can be added in, so as to better deal with any pockets of resistance that survive the opening barrage. Another idea might be to have the RA enter the war with the BL 5.5 inch gun already the main heavy gun, instead of in tandem with and only later replacing the Bl 4.5 inch gun. With better medium-heavy artillery at divsion-level and use of smaller caliber guns in fire plans, this would more than offset the 25 Pounder's lack of shell weight compared to the German and American 105s.