Hermolis--you should be able to place an order for a pickup on the landside, esp as you have 4-6hr connection. The tricky bit is arranging a pickup point. Alternatively, will you have access to airline lounge? I would try calling them, +44 20 8810 4321. One thing I wasn't clear is whether you want them still frozen or chilled--most of the delivery is in chilled form. Also I presume you'll have to check them in due to liquid items.
For all we know, you can even get to Ruben's on Baker St for that much time.
DD's--their website is
http://www.ddsandwiches.com/retail.htm
Note availability at airports can be patchy--sometimes they have it, sometimes they don't stock it. Food is OK, basic--I personally prefer Adafina's sandwiches, though they aren't available at airports.