I suspect 99% of travellers would barely notice the difference. However this is BA Flyertalk so 99% of FTers would notice the difference.

The cabin itself is more or less the same as the LHR product - so seat, food, service levels, bedding and what-not - however you are currently 100% sure of getting low pixel IFE. The screen is big enough but the screen and the underlying IFE technology is well behind the best LHR aircraft (but the same as many of their 777s). So you just don't have the range of films as the 787s/380s etc get.

You are probably more likely to get engaging and experienced cabin crew since LGW very rarely gets MF people and/or crew new to the product. There is no CCR in LGW, but the First lounge is new and is in my view a solid offering. South Terminal has one big payback - its security to lounge to gate experience is fairly compact and hassle free, so if arriving by train you can be in the lounge quickly and easily.

One potential downside is that LGW's longhaul fleet is quite small and if things go wrong there is very little scope to fix things without disrupting the schedule. Mostly of course flights leave on time and to spec.

However I suppose the ace is the destinations since LHR has long list of locations, but LGW has some gems.