Crap.. I just wrote out a long reply and accidentally closed the window! Grr.
Anyway, like the others have said, transferring from Heathrow can be a royal pain in the ass. I know budget's a concern for everyone, but being that I'm in the exact same situation for July (flying into London, then getting over to Barcelona) - and having done this before - here are some considerations beyond the $50-$100 you'll probably save:
* If you're leaving Heathrow to grab another flight elsewhere, you'll have to go through UK passport control. This can take anywhere from a breezy 10 min. to well over 2 hours, depending on how many other flights are coming into your terminal at that hour. Whereas if you take a flight out of LHR, you go through a (relatively) painless international transfer, where you're led to an area with transfer check-in stations and you go on to your next gate.
* Low-cost airlines like RyanAir fly into alternate airports, which can be well outside of the town you want to end up in, meaning another hour or so of transport once you arrive. If you've just been on a trans-atlantic flight, then spent a few hours in immigration and transfer, this could make for a brutally long journey. Personally, as much as I want to save $$$$, I really don't want to arrive at my final destination at 10:00 pm without any sleep the night before.
* BMI (www.flybmi.com
) has some low cost fares that can compete with EasyJet and RyanAir, and they fly out of Heathrow. (Bonus if you come in on Virgin-Atlantic, as they're partners and fly out of the same terminal. You also get Virgin miles!) I got to Dublin for $45 return and Paris for $70 return in the past, without having to trudge over to Gatwick or Stansted. Summer prices are higher, but still a good deal. And if you do the transfer check-in a few hours before the flight, ask for Row 4, and you'll get a business class seat in the economy section
Anyway, my experiences lead me to believe that paying a little extra is the way to go, as much as it hurts at first!