In a nutshell, a change to the handling of ex-Ida has forced changes to the forecast configuration of the jet in mid and west Atlantic. The changes mean a slower progression of features, meaning the front that was due to move in from the Atlantic on Saturday is now held much further west.
Until yesterday, models were split on the speed of progress of the front, with UKMO, Arpege, Canadian and Icon being faster, while GFS and ECMWF favoured the front not reaching the UK. On Tuesday IIRC it was only the GFS that stalled the front, all other models had it progressing into the UK.
The difference of handling of ex-Ida are related to different forecast shapes of an upper trough moving into western North America from the Pacific on Wednesday morning. The downstream impacts from the difference in shape had forecast repercussions right across the USA and the Atlantic. Now the models are in agreement with the handling of the Pacific trough (probably due to better sampling of the upper air in its vicinity now it is over the North American continent) they are coming into line further downstream later in the forecast.
There is still scope for errors in the forecast as the warm air associated with ex-Ida is what is changing the shape of the mid-Atlantic jet, which is what will govern the speed (or lack of) of features moving in from the west over the UK at the weekend.