Should be OK. Forecasting 3-4cm between 1000 Fri & 0800 Sat with low risk of more than that.
Trick is, Mother Nature will decide where she wants to dump the snow and how long she wants to snow for. If the front stalls to the West, then just sleet &/or freezing fog. If it stalls overhead EGLL then it'll be more.
To be fair to BAA, you can't be certain how much snow there'll be and if airlines don't plan properly (de-icing, cancellations due reduced capacity etc) then it'll go bad very quickly.
Let's hope they've got a good plan (& good people to execute it).