1. Improved Matilda II.
Turret ring was the same as the Churchill so fit 6lbr, 75mm and 95mm turrets from Churchill.
The twin engine setup was pretty chunky and complex, while the aft engine deck slopped down, so why not see if a much better engine could fit in the space.
The suspension was bolted to the hull sides under the armoured skirts with its 14" track width, should not be too difficult to replace it with a more durable design and wider track while using spacers as required for the skirts.
End result a much faster, more reliable, heavier hitting and capable medium tank for British and commonwealth forces that improved upon the strengths of the best existing design rather than trying to develop new designs (often already obsolescent) during the height of a battle for survival.
This effective medium tank permits the UK to concentrate on a modern heavy cruiser or universal design much earlier, resulting the deployment of the Centurion in late 44 early 45 as a counter to the Panther / Tiger I. The advent of the Tiger II and Intel on Soviet JSIII etc. leads to the prototyping and experimental deployment of the Conqueror in the last months of the war.
Oh yes of course, Australian licence production of the improved Matilda, maybe using US sourced (or designed) engine, suspension and tracks. Deployment with the 2nd AIF in 40-41 is one regiment of Improved Matilda per division, including the 8th in Malaya and Singapore (although leadership would likely needed to have changed for any real difference).
2. UK adopts a modified version of the CDK TNH (Panzer 38t base vehicle that was actually assessed by the UK pre-war).
An Anglicized version, called the Light Tank Mk X, is selected as the replacement for the various marks of light tanks in service and is in full production by the fall of France and thus becomes the standard British tank for the early years of the war as production is increased to replace the losses of the BEF.
The greater capability of the over the existing light tanks as well as the early cruisers makes it possible for the UK to skip the Crusader, Covenanter, Cavalier and Cromwell, permitting them to instead perfect and deploy the Comet Cruiser, much improved Challenger heavy cruiser and the Avenger tank destroyer in substantial numbers by D Day.