There is a fairly significant historical example of a mainland European nation the UK gave security guarantees to. That turned out to be utterly meaningless in the grand scheme of things, because when that nation's territory was then violated by an aggressor, the UK was left looking like the fool it was for issuing a "guarantee" it had no ability to actually back up.
The UK could do next to nothing about a Russian violation of Finnish sovereignty without significant assistance by the major NATO players, namely the US, and since I doubt that would be forthcoming at best you might be looking at Royal Navy activity in the Baltic sea and Arctic oceans (even then, reluctantly because of the presence of Russian subs and aircraft) and the occasional RAF flyover. Much of that would serve no purpose in any case beyond keeping the Russian navy quiet and their air force on the ground, but as we've seen in Ukraine, Russia is quite content to wage a campaign more or less foregoing those elements in any case so this alone would likely not be enough to actually deter Russian aggression in the first place.
I totally understand both Finland and Sweden's eagerness to join NATO, especially in the aftermath of Russia's aggression toward Ukraine, however, I don't believe the overall NATO response to Finland being invaded would be all that different to the approach to a Russian invasion of a Baltic State. Sucks, but when it comes down to it, nobody is risking WWIII over Finland. There would no doubt be terse words, sanctions, possibly even the sort of military aid we've seen sent to Ukraine, but Finland would likely be fighting the battle itself with little in the way of practical help from NATO.
Regardless though, I think it's a bit moot because Putin will now be well aware of the limitations of Russia's military if he wasn't so prior to 2022, and I don't believe there would be any real zeal in Russia for a potential conflict with Finland in the way there might be with the Baltics. An invasion of Latvia or Estonia would achieve much the same desired effect, and would likely be a far more palatable proposition than another misadventure in Karelia.
Finland is correct to be concerned and correct to be wary though. It's commonly accepted that Sweden's history of actively, visibly, and robustly policing it's own territorial integrity has long been enough to temper any Soviet/Russian ideas about expanding to the Northwest. There is no reason at all to relax that, and in fact, stepping it up as much as viable is precisely what NATO members need to be doing. The best way to approach the threat from Russia right now is to make it as unwelcoming as possible for Russia to contemplate any conventional military expansion of its territory in the first place, not wait for it to happen and then panic about it, because at that point it's already too late because nobody will risk conflagrating and escalating a conflict between nuclear-capable powers.