Betta Fish: Help, advice needed!!

TLDR at the bottom ⬇️

Prefacing this by clarifying I’m in the UK.

Hi everyone, I’m in a bit of a panic here. To sum it all up, I recently bought a 5ish gallon tank and more or less most of the essentials (heater, filter, tap safe etc.) in preparation to buy a betta. I wasn’t intending to actually buy a betta until at least well into December after the tank had been cycled, but my brother thought it was a good idea to “surprise” me with a betta I had previously admired at the pet shop. What can I do to give this fish even a chance at surviving? Genuinely one of the most stunning bettas I’ve seen, but what’s the point in that if he’ll perish instantly. Any advice and tips are much appreciated. Thanks!!

TLDR: Surprised with a betta. Only have an uncycled, not even set up. Have the basics to get it up and running. Need advice.

(Video of the stunner I took when we first visited the pet shop)