Time Out says
You'd be forgiven for assuming hundreds of people dressed as Santa bolting through London is some kind of festive initiation ceremony, but the London Santa Dash is actually a fun run raising money for Great Ormond Street Hospital Children’s Charity. Sign up to sprint a 5km or 10km route in Brockwell Park while wearing red and white threads. Registration includes the Santa suit and mulled wine/mince pie reward at the finish line. If you just fancy heading along to cheer the runners on there'll be a choir and a brass band for entertainment.