That creates the possibility that Germany leaves a token force of the French border, only to be overrun by an initially token French invasion once alt-Barbarossa is underway.
More realistically, Germany would likely have to avoid the MR Pact in any form of this scenario which makes it nearly impossible to prepare for and take on Poland without precipitating conflict with the Soviets.
Indeed.
They
might manage to
force convince Poland to ally itself with Germany (and give away Dantzig in return for the future return of Lithuania) and help in an invasion of the USSR.
Of course, IOTL the Poles refused this proposal as they thought that once the Germans were invited in (as "allies") they would never leave, and they were right, but the Poles feared the Russians just as much if not more, refusing any Russian help even if they were invaded by Germany (which is one of the reasons why Stalin accepted the M-R Pact).
So, in a world where the British (and the French, who aren't daring enough to face Berlin without London's backing) have officially abandoned Poland, the Poles
may accept such a proposal out of despair... knowing that they accept Hitler's friendship "or else".
But even then, invading Soviet Union would be extremely hard. Yes, the Soviets have less years of preparedness, but on the other hand, the Germans have less experience, can't loot the industry and transportation network of France / Low Countries to bolster their campaign logistics, and
don't have surprise effect.
And more importantly, no matter how successful they are in Russia, even if the Soviets are so crippled by the Purges and so unprepared to the novelty of Blitzkrieg that Germans take Moscow, Leningrad and Stalingrad in three months... well, then Germans have France in their backs.
France... and Britain.
I know that OP is about British "Splendid Isolationism", but if the Germans are wildly successful in Russia, the British won't want them to win (they want
both sides to lose), especially if it leads to a giant German industrial empire spanning to the Urals with dozens of millions of slaves and endless raw resources, that will then inevitably build up its navy and air force to challenge the British imperial power. Nope. Regardless of ideological alignment, the British government will prioritize British interests (even -maybe
especially- if it's a far-right nationalist government). And the obvious British interest will
always be to smack down the emerging continental superpower while it's still distracted, and before it's well-established and able to turn west.
So, either the Russians and Germans find themselves in a long grinding war (that bleeds them both, with the French and British delivering weapons to both sides, but always more to the side that seems to be losing, as it wouldn't do if the war ends too quickly...). It's especially likely if the Nazis are wary of a French opportunistic attack, and keep a truly large defence force in the west (which would be a smart choice, but would also mean less available forces in the east...).
Or the Germans have a stunning success and the Heer goes deep into Russia, but then some unlucky
casus belli is found by Paris and London (what about a merchant ship bound to Murmansk that got sunk, and the Western Allies claim it was sunk in international waters...), and stab distracted Germany in the back.