I think you will be ok rhubarb is very hardy, i have large clumps on the allotment that i just leave to brave the elements and they always survive, mine were in the same state as yours in january 10 then we had all that snow, the winter was mild up untill this point and the rhubarb was starting to grow however once the harsh weather had gone the rhubarb did really well, i suppose for peace of mind you could protect with straw etc, hope this helps.

Thank you! If it gets really cold I'll put a bit of straw around, but I think I'll leave it be for now.

You could always cover one root with a large pot and force it. Then if frost threatens you could at least protect the covered one with something. But as Julien says you should be OK whatever happens. If the tops are caught by the frost I'm sure some more would grow later.