The pound is the weakest performer today, falling 1.15% on the first real trading day of the year.

Deutsche Bank notes that there is a pattern here:

Remarkably, this extends sterling's losing run on the first day of the trading year to seven years (i.e. performance on the first trading day after New Year's Day). Over a longer lookback, sterling has only managed three positive returns over the past twenty years of first-day trading
GBP on the first day of the year

Does it extend?

Not at all, DB found no strong pattern over first-day GBP selling extending or reversing in the following week.