I also use the READY data. I use the GFS model (not all models cover the UK) and pick the Default+winds version (usually the 84 hour model).
The big problem I see is that when the READY model says that the wind direction will change significantly a couple of days out, WxSim wants to use MOS data to update it’s advection info. I don’t know what MOS data is, but I do know it isn’t available in the UK. At this point I import the (regional) Synop data for the new wind direction, but I’m not sure whether this is sensible or not because the winds over the intervening couple of days would probably have changed the conditions from the regional stations I’m now using. This is where a bigger model works better because I assume it would have worked out the effects over time of changing conditions in numerous cells rather than using advection over longer distances/periods.