It's been mutating for months and months. @Tubey made that point a week or so ago that they've found new strains in plenty of places.
All our scientists have done is find the new strain, and now everywhere in the world will obviously start looking for it - and start picking it up.
I'm mad that our government are using it as an excuse for further restrictions, whereas really all that knackered them up was them undermining their own Tier system by not putting the capital and surrounding areas into tougher restrictions a month back.
The infections/deaths rising is what will drive new restrictions. Obviously.