Travelling over new year in Japan

Hi, we are three adults travelling from the UK to Japan for snow sports.

Arriving in Tokyo 28 December.

Leaving 29 December for Hakuba

Leaving 4 January for Kyoto

Leaving 6 January for Tokyo

7 January return to UK

I have heard that trains in Japan over new year are very busy and that reserving a seat is required.

However if we buy a JR pass in the UK we cannot validate the pass, and cannot reserve seats, until we reach Japan, and then it will be too late.

Is it better to make this trip with a JR pass, or could we buy individual tickets for each part of the journey?

Would this be a cheaper way to travel?

If we buy individual tickets can we reserve seats in advance from the UK?

Thank you for your assistance.