Ordinarily, I'd recommend you allow at least 2-3 weeks, but it depends whether you are ordering a "move" of broadband from an existing house, or a fresh connection at the new house.
We ordered a move from our old house with 2 weeks notice, and had complications in getting a simultaneous-provide done for fibre. In the end we were stuck with just phone for a few days, then got ADSL for a couple of weeks, then got the fibre.
However, that was to a house which didn't have anyone using the existing line at the time I ordered. Having someone in the house, and using Sky LLU, is likely to add something of a complication.