Mini Cooper dead as a dodo

[Leggy]

Missus has a '63 plate Cooper. Tried to start it this morning and completely dead. The hazard light flickered a bit but otherwise nothing.
Been trouble free otherwise, any ideas appreciated.

[Leggy]

Disconnected and charged battery as trickle charging made no difference. Car now starts. However been advised by local garage that we might need a new battery but the car will then have to be Factory/ garage reset with the battery registered?? Is this just BS, why can't I just fit my own battery and carry on my merry way like the old days!

[Leggy]

Halfords do a battery for £150 and can install/set up for about £25 allegedly. Any experience as begrudge paying dealer prices, as they have you over a barrel.