I would recommend the KLM-flights from SPL.
KLM's C-class is pretty good to very good, prices competative, frequency of flights to the USA is very high, especially with partners included.
Cabins are all fairly new and have good IFE.
It surely beats the hassle of changing terminals at LHR where i have seen many suitcases disappear during transfers.....
SPL is a good place to stay if necessary, lounges are comfortable and no inbetween terminal travels needed.
I have flown C-class (medium- and long haul) on alot of different (European) airlines but feel KLM does offer best value for money and (most) crews are very friendly and efficient.