You can get Hermo sandwiches at BA Club lounge at T5, but you'll need to call a few days ahead. I didn't get a full meal though.
4hrs would be tight connex to come all the way to pick up something from WestEnd. One alternative that MIGHT work is Westfield shopping centre near Shephard's Bush. (I'd say 30min or so by taxi) There's supposed to be a take-away concession of Isola Bella (kosher restaurant in Hendon). Though it's a BIG mall and traffic can get really crowded on Sundays.
As for prepackaged sandwiches, I personally think Adafina ones are decent, especially bread compared to the other brand. Admittedly the sizes are small, though price-wise they're comparable to non-kosher varieties. Though agree Adafina's better with their salad/pasta boxes (chicken couscous is my favourite, esp that red sauce).