Once your subscription expires Hitfilm will revert to Hitfilm Free and creator/pro tier features will be watermarked.
I'm sorry, I don't remember offhand where the buy Perpetual button is on the site. I believe it's discussed in this thread.
I believe a new Perpetual license is $399. This gets you one year of updates, then you version lock. Renewals of update licenses are cheaper.
No. You will be able to use that license forever. However, after 12 months you can't get any new updates.
For sake of discussion, the current version of Hitfilm is 2022.2. Let's say you buy a perpetual Pro license. You get 12 months of updates. Let's say by next October you have "Hitfilm 2023.3" when your license ends. You can continue to use Hitfilm Pro 2023.3 forever, but could not update to a future "Hitfilm Pro 2024.1" unless you buy another 12 months of updates (which will be less than £309), but you'd still be running Hitfilm PRO (no watermarks on anything) . Where, with a subscription Hitfilm PRO ending your subscription means your Hitfilm would revert to Hitfilm FREE (watermarks on Creator or Pro features).
Hope I've explained this well.