The battery needs to be a 019, AGM the larger the better. Do not buy a 020 they do of it but are very tight and not recommended.
You reset the clock and date etc via the screen it’s straight forward….have a look in your handbook. 2012 tdv8 Westminster arrived Jan 19 gone Dec 23

200 TDi discovery bobtail gone
RR classic v8 bobtail gone
300 tdi Discovery 3 dr gone

As long you don't get the 'Press brake to start' info on the instrument panel when you press the start button you're pressing the brake pedal far enough to reach the starter safety switch.
Simple as that.

The brake pedal losing vacuum when not running the engine for a day or two and making it kinda hard to push in as soon as you try is not completely normal in my opinion, but still not related to not having enough battery power to start.

Reading OP's post again I see they pressed the brake pedal a couple of times trying to start.
The brake pedal firms up doing that (without a running engine), normal in my opinion.
The engine ticking over is proof enough the starter safety switch was reached.

And I agree with V8 Bob about the battery size.
Size 020 did fit in my previous 3.6TDV8 - with a LOT of effort - but I didn't even try with my 2012 4.4TDV8.
AGM battery is recommended!
My battery guy said to get the Varta AGM, he had no customers ever coming back with those as he had with Bosch, I had a Bosch AGM before that lasted only 5 years.
But maybe 5 years is normal for a battery nowadays. 2012 4.4TDV8
2000 Honda S2000