I checked my Rough-legged Buzzard records and found them equally divided about the turn of the year. I suspect there is observer bias in my case persuading me to make more effort at the end of the year if I haven't seen one near the start. The following are after Jan 1:Looks fine for Common Buzzard.
I should add, a Rough-legged Buzzard would be fairly unlikely this time of the year - most records that I can think of (from one’s that I’ve seen in Norfolk anyway) have been in the Autumn - Common Buzzards are hugely variable with some quite pale, so ruling out CB first before considering Rough-legged is the safest approach. Of course, if you see a Buzzard perched, check the legs( tarsi) to see if they are feathered or unfeathered - if the latter, you have a Common Buzzard without worrying about plumage.
In the air, look out for hovering Buteos with strangely twisty postures and dark bendy elbows!
Massingham Heath, Norfolk, 19 Mar 1989
Massingham Heath, Norfolk, 4 Mar 1990
Fingringhoe Wick, Essex, 20 Jan 1991
West Dean, West Sussex, 17 Feb 1996
Harty Marshes, Sheppey, Kent, 10 Jan 1998
Harty Marshes, Sheppey, Kent, 24 Jan 1999
Massingham Heath, Norfolk, 13 Feb 1999
Haxton Down, Wiltshire, 9 Jan 2000
Massingham Heath, Norfolk, 22 Feb 2004
Harty Marshes, Sheppey, Kent, 5 Mar 2006 (Marion)
The Burgh, West Sussex, 27 Nov 2011 and into 2012 P
Jevington, East Sussex, 29 Nov 2014 and 4 January 2015 P
Lochindorb, Inverness-shire, 7 Apr 2015
Rough-legged Buzzards in Britain are not just migrants but winter visitors and as such remain into early spring. Even in Norfolk.