..........
On the ground Matilda IIs, which are contemporary with Pz.Kpfw IVs & only a little later than Pz.Kpfw IIIs, would have suited the Spanish terrain & have been a significant challenge for the Axis tanks operating in Spain, being almost invulnerable to their guns & having a gun capable of destroying them. "Allied" involvement would, also, have given them experience against some early iterations of what became "Blitzkrieg".
So if the Germans and Italians encountered the Matilda II in the Spanish Civil War, there's every chance that they would have enacted the development and equipping of a larger and higher velocity gun for the likes of the Pz.Kpfw III and Pz.Kpfw IV earlier, which would have put the Germans in a far better position to combat both the French B1 bis, Matilda II, to say nothing of the Soviet T-34 during the prevailing Second World War.
I'm guessing it would work both ways, with the British hopefully also learning the shortfalls of their Matilda II's puny QF 2pdr (40mm) gun - especially it's dismal lack of a HE round. Perhaps, spurring on a more urgent development and fitting of the QF 3pdr (47mm) gun or even better and more practical, a 6-pdr (57 mm) high-velocity gun!
MAD
It was Wehrmacht doctrine to use the 8.8cm (88mm) in the AT role well before 1939, & it was a very effective Matilda II killer. It was just a matter of them not being in the right place at the right time.
In France; it was because they were mostly left behind in the name of speed. In Africa; good recce work & good luck meant the Matildas rarely ran into them (but, when they did, it wasn't pretty).
Thanks for your feedback Old Wimbat.
I'm very aware of the improvised adoption of the 8.8cm Flak gun as an anti-tank weapon, but it was an improvisation through necessity vs design. Although irrufutably effective, the improvised employment of the 8.8cm Flak was also ungainly unquestionably ungainly due to it's anti-aircraft design/layout. Hence why I was alluding to a by-design vs improvisation.
MAD
I don't know where people get the idea using the 8.8cm was "improvised" (generally, by Rommell's troops during the desert campaign}.
The Germans
trained their 8.8cm gunners &
supplied them with AT ammunition from at least the early mid-1930's & they were formed in self-supporting AT formations just behind the front lines.
It was
doctrine, therefore
far from improvised.
The British could have done exactly the same thing with their 3.7" AA guns (93.98mm, so say 94mm) just as easily & more effectively (bigger gun, faster projectile) if they'd had the imagination.